Revert "Revert 182527" ([Sync] Make SESSIONS an implicit type)

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 352 matching lines @@
   // for the traffic reported in this ChromiumExtensionsActivity.
   optional string extension_id = 1;
 
   // How many times the extension successfully invoked a write
   // operation through the bookmarks API since the last CommitMessage.
   optional uint32 bookmark_writes_since_last_commit = 2;
 };
 
 // Client specific configuration information.
 message ClientConfigParams {
-  // The set of data types this client has enabled.
+  // The set of data types this client has enabled. Note that this does not
+  // include proxy types, as they do not have protocol field numbers and are
+  // placeholder types that implicitly enable protocol types.
  repeated int32 enabled_type_ids = 1;
+
+ // Whether the PROXY_TABS proxy datatype is enabled on this client.
+ optional bool tabs_datatype_enabled = 2;
 };
 
 message CommitMessage {
   repeated SyncEntity entries = 1;
 
   // A GUID that identifies the committing sync client.  This value will be
   // returned as originator_cache_guid for any new items.
   optional string cache_guid = 2;
 
   repeated ChromiumExtensionsActivity extensions_activity = 3;
@@ skipping 375 matching lines @@
     // is absent then the whole client (all datatypes) is throttled.
     repeated int32 error_data_type_ids = 5;
   }
   optional Error error = 13;
 
   // The new per-client state for this client. If set, should be persisted and
   // sent with any subsequent ClientToServerMessages.
   optional ChipBag new_bag_of_chips = 14;
 };