[Sync] Add support for performing a GetKey on startup.

sync/protocol/sync.proto

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // Sync protocol for communication between sync client and server.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
@@ skipping 428 matching lines @@
   repeated DataTypeProgressMarker from_progress_marker = 6;
 
   // Indicates whether the response should be sent in chunks.  This may be
   // needed for devices with limited memory resources.  If true, the response
   // will include one or more ClientToServerResponses, with the frist one
   // containing GetUpdatesMetadataResponse, and the remaining ones, if any,
   // containing GetUpdatesStreamingResponse.  These ClientToServerResponses are
   // delimited by a length prefix, which is encoded as a varint.
   optional bool streaming = 7 [default = false];
 
+  // Whether the client needs the server to provide an encryption key for this
+  // account.
+  // Note: this should typically only be set on the first GetUpdates a client
+  // requests. Clients are expected to persist the encryption key from then on.
+  // The allowed frequency for requesting encryption keys is much lower than
+  // other datatypes, so repeated usage will likely result in throttling.
+  optional bool need_encryption_key = 8 [default = false];
+
   // Whether to create the mobile bookmarks folder if it's not
   // already created.  Should be set to true only by mobile clients.
   optional bool create_mobile_bookmarks_folder = 1000 [default = false];
 };
 
 message AuthenticateMessage {
   required string auth_token = 1;
 };
 
 message ClientToServerMessage {
@@ skipping 106 matching lines @@
   // the batch, or if there were no new updates on the server; and the client
   // must save these.  If the server does not provide a |new_progress_marker|
   // value for a particular datatype, when the request provided a
   // |from_progress_marker| value for that datatype, the client should
   // interpret this to mean "no change from the previous state" and retain its
   // previous progress-marker value for that datatype.
   //
   // Progress markers in the context of a response will never have the
   // |timestamp_token_for_migration| field set.
   repeated DataTypeProgressMarker new_progress_marker = 5;
+
+  // The current encryption key associated with this account. Only set if the
+  // GetUpdatesMessage in the request had need_encryption_key == true.
+  optional bytes encryption_key = 6;
 };
 
 // The metadata response for GetUpdatesMessage.  This response is sent when
 // streaming is set to true in the request.  It is prefixed with a length
 // delimiter, which is encoded in varint.
 message GetUpdatesMetadataResponse {
   // Approximate count of changes remaining.  Detailed comment is available in
   // GetUpdatesResponse.
   optional int64 changes_remaining = 1;
 
@@ skipping 33 matching lines @@
   required double target_utilization = 2;
   required double measure_interval_max = 3;
   required double measure_interval_min = 4;
   required double observation_window = 5;
 };
 
 message ClientToServerResponse {
   optional CommitResponse commit = 1;
   optional GetUpdatesResponse get_updates = 2;
   optional AuthenticateResponse authenticate = 3;
-  // DEPRECATED - this field was never used in production.
-  // optional ClearUserDataResponse clear_user_data = 9;
-  optional GetUpdatesMetadataResponse stream_metadata = 10;
-  // If GetUpdatesStreamingResponse is contained in the ClientToServerResponse,
-  // none of the other fields (error_code and etc) will be set.
-  optional GetUpdatesStreamingResponse stream_data = 11;
-
-  message Error {
-    optional SyncEnums.ErrorType error_type = 1 [default = UNKNOWN];
-    optional string error_description       = 2;
-    optional string url                     = 3;
-    optional SyncEnums.Action action        = 4 [default = UNKNOWN_ACTION];
-
-    // Currently only meaningful if |error_type| is throttled. If this field
-    // is absent then the whole client (all datatypes) is throttled.
-    repeated int32 error_data_type_ids = 5;
-  }
-
-  optional Error error = 13;
 
   // Up until protocol_version 24, the default was SUCCESS which made it
   // impossible to add new enum values since older clients would parse any
   // out-of-range value as SUCCESS. Starting with 25, unless explicitly set,
   // the error_code will be UNKNOWN so that clients know when they're
   // out-of-date. Note also that when using protocol_version < 25,
   // TRANSIENT_ERROR is not supported. Instead, the server sends back a HTTP
   // 400 error code. This is deprecated now.
   optional SyncEnums.ErrorType error_code = 4 [default = UNKNOWN];
   optional string error_message = 5;
 
   // Opaque store ID; if it changes, the contents of the client's cache
   // is meaningless to this server.  This happens most typically when
   // you switch from one storage backend instance (say, a test instance)
   // to another (say, the official instance).
   optional string store_birthday = 6;
 
   optional ClientCommand client_command = 7;
   optional ProfilingData profiling_data = 8;
+  // DEPRECATED - this field was never used in production.
+  // optional ClearUserDataResponse clear_user_data = 9;
+  optional GetUpdatesMetadataResponse stream_metadata = 10;
+  // If GetUpdatesStreamingResponse is contained in the ClientToServerResponse,
+  // none of the other fields (error_code and etc) will be set.
+  optional GetUpdatesStreamingResponse stream_data = 11;
 
   // The data types whose storage has been migrated.  Present when the value of
   // error_code is MIGRATION_DONE.
   repeated int32 migrated_data_type_id = 12;
+
+  message Error {
+    optional SyncEnums.ErrorType error_type = 1 [default = UNKNOWN];
+    optional string error_description       = 2;
+    optional string url                     = 3;
+    optional SyncEnums.Action action        = 4 [default = UNKNOWN_ACTION];
+
+    // Currently only meaningful if |error_type| is throttled. If this field
+    // is absent then the whole client (all datatypes) is throttled.
+    repeated int32 error_data_type_ids = 5;
+  }
+  optional Error error = 13;
 };