[Sync] Convert NonBlockingDataTypeController to be single-thread.

chrome/browser/sync/chrome_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 CHROME_BROWSER_SYNC_CHROME_SYNC_CLIENT_H__
 #define CHROME_BROWSER_SYNC_CHROME_SYNC_CLIENT_H__
 
 #include <vector>
 
 #include "base/macros.h"
@@ skipping 33 matching lines @@
   base::Closure GetPasswordStateChangedCallback() override;
   sync_driver::SyncApiComponentFactory::RegisterDataTypesMethod
   GetRegisterPlatformTypesCallback() override;
   autofill::PersonalDataManager* GetPersonalDataManager() override;
   invalidation::InvalidationService* GetInvalidationService() override;
   BookmarkUndoService* GetBookmarkUndoServiceIfExists() 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;
   sync_driver::SyncApiComponentFactory* GetSyncApiComponentFactory() override;
 
   // Helpers for overriding getters in tests.
   void SetSyncApiComponentFactoryForTesting(
       std::unique_ptr<sync_driver::SyncApiComponentFactory> component_factory);
 
@@ skipping 34 matching lines @@
   ExtensionsActivityMonitor extensions_activity_monitor_;
 
   base::WeakPtrFactory<ChromeSyncClient> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ChromeSyncClient);
 };
 
 }  // namespace browser_sync
 
 #endif  // CHROME_BROWSER_SYNC_CHROME_SYNC_CLIENT_H__