[Sync] Convert NonBlockingDataTypeController to be single-thread.

components/sync/driver/fake_sync_client.h

 // Copyright 2015 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.
 
 #ifndef COMPONENTS_SYNC_DRIVER_FAKE_SYNC_CLIENT_H_
 #define COMPONENTS_SYNC_DRIVER_FAKE_SYNC_CLIENT_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 20 matching lines @@
   base::Closure GetPasswordStateChangedCallback() override;
   sync_driver::SyncApiComponentFactory::RegisterDataTypesMethod
   GetRegisterPlatformTypesCallback() override;
   autofill::PersonalDataManager* GetPersonalDataManager() override;
   BookmarkUndoService* GetBookmarkUndoServiceIfExists() override;
   invalidation::InvalidationService* GetInvalidationService() override;
   scoped_refptr<syncer::ExtensionsActivity> GetExtensionsActivity() override;
   sync_sessions::SyncSessionsClient* GetSyncSessionsClient() override;
   base::WeakPtr<syncer::SyncableService> GetSyncableServiceForType(
       syncer::ModelType type) override;
-  syncer_v2::ModelTypeService* GetModelTypeServiceForType(
+  base::WeakPtr<syncer_v2::ModelTypeService> GetModelTypeServiceForType(
       syncer::ModelType type) override;
   scoped_refptr<syncer::ModelSafeWorker> CreateModelWorkerForGroup(
       syncer::ModelSafeGroup group,
       syncer::WorkerLoopDestructionObserver* observer) override;
   SyncApiComponentFactory* GetSyncApiComponentFactory() override;
 
   void SetModelTypeService(syncer_v2::ModelTypeService* model_type_service);
 
  private:
   syncable_prefs::TestingPrefServiceSyncable pref_service_;
   syncer_v2::ModelTypeService* model_type_service_;
   SyncApiComponentFactory* factory_;
   std::unique_ptr<FakeSyncService> sync_service_;
 
   DISALLOW_COPY_AND_ASSIGN(FakeSyncClient);
 };
 
 }  // namespace sync_driver
 
 #endif  // COMPONENTS_SYNC_DRIVER_FAKE_SYNC_CLIENT_H_