[Sync] Add favicon sync experiment

sync/protocol/experiments_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 experimental feature flags.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 option retain_unknown_fields = true;
 
 package sync_pb;
 
 // A flag to enable support for keystore encryption.
 message KeystoreEncryptionFlags {
   optional bool enabled = 1;
 }
 
 // Whether history delete directives are enabled.
 message HistoryDeleteDirectives {
   optional bool enabled = 1;
 }
 
 // Whether this client should cull (delete) expired autofill
 // entries when autofill sync is enabled.
 message AutofillCullingFlags {
   optional bool enabled = 1;
 }
 
+// Whether the favicon sync datatypes are enabled.
+message FaviconSyncFlags {
+  optional bool enabled = 1;
+}
+
 // Contains one flag or set of related flags.  Each node of the experiments type
 // will have a unique_client_tag identifying which flags it contains.  By
 // convention, the tag name should match the sub-message name.
 message ExperimentsSpecifics {
   optional KeystoreEncryptionFlags keystore_encryption = 1;
   optional HistoryDeleteDirectives history_delete_directives = 2;
   optional AutofillCullingFlags autofill_culling = 3;
+  optional FaviconSyncFlags favicon_sync = 4;
 }