[SyncFS] Make ExtensionService usage of SyncWorker asynchronous

chrome/browser/sync_file_system/drive_backend/sync_engine.cc

 // Copyright 2013 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_file_system/drive_backend/sync_engine.h"
 
 #include <vector>
 
 #include "base/bind.h"
 #include "base/threading/sequenced_worker_pool.h"
@@ skipping 175 matching lines @@
   scoped_ptr<SyncEngineContext> sync_engine_context(
       new SyncEngineContext(drive_service_.get(),
                             drive_uploader_.get(),
                             base::MessageLoopProxy::current(),
                             worker_task_runner_,
                             file_task_runner));
   worker_observer_.reset(
       new WorkerObserver(base::MessageLoopProxy::current(),
                          weak_ptr_factory_.GetWeakPtr()));
 
+  base::WeakPtr<ExtensionServiceInterface> extension_service_weak_ptr;
+  if (extension_service_)
+    extension_service_weak_ptr = extension_service_->AsWeakPtr();
+
   // TODO(peria): Use PostTask on |worker_task_runner_| to call this function.
   sync_worker_ = SyncWorker::CreateOnWorker(
       base_dir,
       worker_observer_.get(),
-      extension_service_,
+      extension_service_weak_ptr,
       sync_engine_context.Pass(),
       env_override);
 
   if (notification_manager_)
     notification_manager_->AddObserver(this);
   GetDriveService()->AddObserver(this);
   net::NetworkChangeNotifier::AddNetworkChangeObserver(this);
 }
 
 void SyncEngine::AddServiceObserver(SyncServiceObserver* observer) {
@@ skipping 307 matching lines @@
         tracker.tracker_kind() == TRACKER_KIND_APP_ROOT;
     if (is_app_enabled && !is_app_root_tracker_enabled)
       EnableOrigin(origin, base::Bind(&EmptyStatusCallback));
     else if (!is_app_enabled && is_app_root_tracker_enabled)
       DisableOrigin(origin, base::Bind(&EmptyStatusCallback));
   }
 }
 
 }  // namespace drive_backend
 }  // namespace sync_file_system