[Sync] Convert NonBlockingDataTypeController to be single-thread.

chrome/browser/sync/chrome_sync_client.cc

 // 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.
 
 #include "chrome/browser/sync/chrome_sync_client.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/command_line.h"
@@ skipping 407 matching lines @@
 #endif
     default:
       // The following datatypes still need to be transitioned to the
       // syncer::SyncableService API:
       // Bookmarks
       NOTREACHED();
       return base::WeakPtr<syncer::SyncableService>();
   }
 }
 
-syncer_v2::ModelTypeService* ChromeSyncClient::GetModelTypeServiceForType(
-    syncer::ModelType type) {
+base::WeakPtr<syncer_v2::ModelTypeService>
+ChromeSyncClient::GetModelTypeServiceForType(syncer::ModelType type) {
   switch (type) {
     case syncer::DEVICE_INFO:
       return ProfileSyncServiceFactory::GetForProfile(profile_)
-          ->GetDeviceInfoService();
+          ->GetDeviceInfoService()
+          ->AsWeakPtr();
     default:
       NOTREACHED();
-      return nullptr;
+      return base::WeakPtr<syncer_v2::ModelTypeService>();
   }
 }
 
 scoped_refptr<syncer::ModelSafeWorker>
 ChromeSyncClient::CreateModelWorkerForGroup(
     syncer::ModelSafeGroup group,
     syncer::WorkerLoopDestructionObserver* observer) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   switch (group) {
     case syncer::GROUP_DB:
@@ skipping 172 matching lines @@
   sync_service->RegisterDataTypeController(
       new SupervisedUserSyncDataTypeController(syncer::SUPERVISED_USER_SETTINGS,
                                                error_callback, this, profile_));
   sync_service->RegisterDataTypeController(
       new SupervisedUserSyncDataTypeController(
           syncer::SUPERVISED_USER_WHITELISTS, error_callback, this, profile_));
 #endif
 }
 
 }  // namespace browser_sync