[Sync] Stop accessing BrowserContextKeyedServiceFactory on non-UI thread.

ios/chrome/browser/sync/ios_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 IOS_CHROME_BROWSER_SYNC_IOS_CHROME_SYNC_CLIENT_H__
 #define IOS_CHROME_BROWSER_SYNC_IOS_CHROME_SYNC_CLIENT_H__
 
 #include <memory>
 #include <vector>
 
@@ skipping 36 matching lines @@
   history::HistoryService* GetHistoryService() override;
   bool HasPasswordStore() override;
   base::Closure GetPasswordStateChangedCallback() override;
   syncer::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;
+  ServiceProvider GetSyncableServiceForType(syncer::ModelType type) override;
   base::WeakPtr<syncer::ModelTypeSyncBridge> GetSyncBridgeForModelType(
       syncer::ModelType type) override;
   scoped_refptr<syncer::ModelSafeWorker> CreateModelWorkerForGroup(
       syncer::ModelSafeGroup group) override;
   syncer::SyncApiComponentFactory* GetSyncApiComponentFactory() override;
 
   void SetSyncApiComponentFactoryForTesting(
       std::unique_ptr<syncer::SyncApiComponentFactory> component_factory);
 
   // Iterates over browser states and returns any trackers that can be found.
@@ skipping 12 matching lines @@
   scoped_refptr<password_manager::PasswordStore> password_store_;
 
   std::unique_ptr<sync_sessions::SyncSessionsClient> sync_sessions_client_;
 
   base::WeakPtrFactory<IOSChromeSyncClient> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(IOSChromeSyncClient);
 };
 
 #endif  // IOS_CHROME_BROWSER_SYNC_IOS_CHROME_SYNC_CLIENT_H__