[Sync] Add favicon datatype proto support

sync/protocol/favicon_tracking_specifics.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 datatype extension for the favicon tracking type.
+
+syntax = "proto2";
+
+option optimize_for = LITE_RUNTIME;
+option retain_unknown_fields = true;
+
+package sync_pb;
+
+// Tracking info for of favicon images. These control expiration of images
+// from sync based on recency, bookmark state, etc.
+message FaviconTrackingSpecifics {
+  // The url of the favicon image.
+  optional string favicon_url= 1;
+  // The last time a page using this favicon was visited (in milliseconds
+  // since linux epoch).
+  optional int64 last_visit_time_ms = 3;
+  // Whether this favicon is currently bookmarked or not.
+  optional bool is_bookmarked = 4;
+}