[SyncFS] Construct and destruct sync_worker in worker_task_runner

chrome/browser/sync_file_system/drive_backend/sync_worker.h

 // Copyright 2014 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 CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_SYNC_WORKER_H_
 #define CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_SYNC_WORKER_H_
 
 #include <set>
 #include <string>
 
@@ skipping 43 matching lines @@
   enum AppStatus {
     APP_STATUS_ENABLED,
     APP_STATUS_DISABLED,
     APP_STATUS_UNINSTALLED,
   };
 
   typedef base::hash_map<std::string, AppStatus> AppStatusMap;
 
   class Observer {
    public:
+    virtual ~Observer() {}
+
     virtual void OnPendingFileListUpdated(int item_count) = 0;
     virtual void OnFileStatusChanged(const fileapi::FileSystemURL& url,
                                      SyncFileStatus file_status,
                                      SyncAction sync_action,
                                      SyncDirection direction) = 0;
     virtual void UpdateServiceState(RemoteServiceState state,
                                     const std::string& description) = 0;
-
-   protected:
-    virtual ~Observer() {}
   };
 
   static scoped_ptr<SyncWorker> CreateOnWorker(
       const base::FilePath& base_dir,
       Observer* observer,
       const base::WeakPtr<ExtensionServiceInterface>& extension_service,
       scoped_ptr<SyncEngineContext> sync_engine_context,
       leveldb::Env* env_override);
 
+  static void DestructOnWorker(scoped_ptr<SyncWorker> sync_worker,
+                               scoped_ptr<Observer> worker_observer);
+
   virtual ~SyncWorker();
 
   void Initialize();
 
   // SyncTaskManager::Client overrides
   virtual void MaybeScheduleNextTask() OVERRIDE;
   virtual void NotifyLastOperationStatus(
       SyncStatusCode sync_status, bool used_network) OVERRIDE;
 
   void RegisterOrigin(const GURL& origin, const SyncStatusCallback& callback);
@@ skipping 103 matching lines @@
   ObserverList<Observer> observers_;
 
   base::WeakPtrFactory<SyncWorker> weak_ptr_factory_;
   DISALLOW_COPY_AND_ASSIGN(SyncWorker);
 };
 
 }  // namespace drive_backend
 }  // namespace sync_file_system
 
 #endif  // CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_SYNC_WORKER_H_