[Sync] Add support for retrying a getupdates due to a context change

sync/test/engine/mock_update_handler.cc

 // Copyright 2014 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.
 
 #include "sync/test/engine/mock_update_handler.h"
 
 #include "sync/internal_api/public/base/model_type.h"
 
 namespace syncer {
 
@@ skipping 11 matching lines @@
 void MockUpdateHandler::GetDownloadProgress(
       sync_pb::DataTypeProgressMarker* progress_marker) const {
   progress_marker->CopyFrom(progress_marker_);
 }
 
 void MockUpdateHandler::GetDataTypeContext(
     sync_pb::DataTypeContext* context) const {
   context->Clear();
 }
 
-void MockUpdateHandler::ProcessGetUpdatesResponse(
+SyncerError MockUpdateHandler::ProcessGetUpdatesResponse(
     const sync_pb::DataTypeProgressMarker& progress_marker,
     const sync_pb::DataTypeContext& mutated_context,
     const SyncEntityList& applicable_updates,
     sessions::StatusController* status) {
   progress_marker_.CopyFrom(progress_marker);
+  return syncer::SYNCER_OK;
 }
 
 void MockUpdateHandler::ApplyUpdates(sessions::StatusController* status) {
   apply_updates_count_++;
 }
 
 void MockUpdateHandler::PassiveApplyUpdates(
     sessions::StatusController* status) {
   passive_apply_updates_count_++;
 }
 
 int MockUpdateHandler::GetApplyUpdatesCount() {
   return apply_updates_count_;
 }
 
 int MockUpdateHandler::GetPassiveApplyUpdatesCount() {
   return passive_apply_updates_count_;
 }
 
 }  // namespace syncer