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

components/sync/driver/directory_data_type_controller.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_DIRECTORY_DATA_TYPE_CONTROLLER_H__
 #define COMPONENTS_SYNC_DRIVER_DIRECTORY_DATA_TYPE_CONTROLLER_H__
 
 #include <memory>
 
 #include "components/sync/driver/data_type_controller.h"
@@ skipping 55 matching lines @@
   // Create an error handler that reports back to this controller.
   virtual std::unique_ptr<DataTypeErrorHandler> CreateErrorHandler() = 0;
 
   // Access to the ChangeProcessor for the type being controlled by |this|.
   // Returns null if the ChangeProcessor isn't created or connected.
   virtual ChangeProcessor* GetChangeProcessor() const = 0;
 
   // Function to capture and upload a stack trace when an error occurs.
   base::Closure dump_stack_;
 
+  // Non-owning pointer, should only be accessed on the UI thread.
   SyncClient* const sync_client_;
 
  private:
   // The model safe group of this data type.  This should reflect the
   // thread that should be used to modify the data type's native
   // model.
   ModelSafeGroup model_safe_group_;
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_DRIVER_DIRECTORY_DATA_TYPE_CONTROLLER_H__