[Sync] Convert NonBlockingDataTypeController to be single-thread.

components/sync/driver/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_SYNC_CLIENT_H_
 #define COMPONENTS_SYNC_DRIVER_SYNC_CLIENT_H_
 
 #include "base/callback_forward.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
@@ skipping 75 matching lines @@
   virtual base::Closure GetPasswordStateChangedCallback() = 0;
 
   virtual autofill::PersonalDataManager* GetPersonalDataManager() = 0;
   virtual BookmarkUndoService* GetBookmarkUndoServiceIfExists() = 0;
   virtual invalidation::InvalidationService* GetInvalidationService() = 0;
   virtual scoped_refptr<syncer::ExtensionsActivity> GetExtensionsActivity() = 0;
   virtual sync_sessions::SyncSessionsClient* GetSyncSessionsClient() = 0;
 
   // Returns a weak pointer to the syncable service specified by |type|.
   // Weak pointer may be unset if service is already destroyed.
-  // Note: Should only be called from the model type thread.
+  // Note: Should only be dereferenced from the model type thread.
   virtual base::WeakPtr<syncer::SyncableService> GetSyncableServiceForType(
       syncer::ModelType type) = 0;
 
-  // Returns a non-owning pointer to the service specified by |type|. Service
-  // lifetime is independent from sync thread therefore pointer should not be
-  // retained across tasks.
-  // Note: Should only be called from the model type thread.
-  // Note: should only be called by USS.
-  virtual syncer_v2::ModelTypeService* GetModelTypeServiceForType(
+  // Returns a weak pointer to the ModelTypeService specified by |type|. Weak
+  // pointer may be unset if service is already destroyed.
+  // Note: Should only be dereferenced from the model type thread.
+  virtual base::WeakPtr<syncer_v2::ModelTypeService> GetModelTypeServiceForType(
       syncer::ModelType type) = 0;
 
   // Creates and returns a new ModelSafeWorker for the group, or null if one
   // cannot be created.
   // TODO(maxbogue): Move this inside SyncApiComponentFactory.
   virtual scoped_refptr<syncer::ModelSafeWorker> CreateModelWorkerForGroup(
       syncer::ModelSafeGroup group,
       syncer::WorkerLoopDestructionObserver* observer) = 0;
 
   // Returns the current SyncApiComponentFactory instance.
   virtual SyncApiComponentFactory* GetSyncApiComponentFactory() = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(SyncClient);
 };
 
 }  // namespace sync_driver
 
 #endif  // COMPONENTS_SYNC_DRIVER_SYNC_CLIENT_H_