[Sync] Add support for automatic enabling of syncing tab favicons.

sync/protocol/nigori_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 nigori keys.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
@@ skipping 72 matching lines @@
   // If true, all current and future datatypes will be encrypted.
   optional bool encrypt_everything = 24;
 
   optional bool encrypt_extension_settings = 25;
   optional bool encrypt_app_notifications = 26;
   optional bool encrypt_app_settings = 27;
 
   // User device information. Contains information about each device that has a
   // sync-enabled Chrome browser connected to the user account.
   repeated DeviceInformation device_information = 28;
+  
+  // Enable syncing favicons as part of tab sync.
+  optional bool sync_tab_favicons = 29;
 }