[SyncFS] Remove usage of file task runner under drive_backend/

chrome/browser/sync_file_system/drive_backend/sync_engine.h

 // 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.
 
 #ifndef CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_SYNC_ENGINE_H_
 #define CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_SYNC_ENGINE_H_
 
 #include <set>
 #include <string>
 
@@ skipping 130 matching lines @@
   virtual void GoogleSignedOut(const std::string& username) OVERRIDE;
 
  private:
   class WorkerObserver;
 
   friend class DriveBackendSyncTest;
   friend class SyncEngineTest;
 
   SyncEngine(base::SingleThreadTaskRunner* ui_task_runner,
              base::SequencedTaskRunner* worker_task_runner,
-             base::SequencedTaskRunner* file_task_runner,
              base::SequencedTaskRunner* drive_task_runner,
              const base::FilePath& sync_file_system_dir,
              TaskLogger* task_logger,
              drive::DriveNotificationManager* notification_manager,
              ExtensionServiceInterface* extension_service,
              SigninManagerBase* signin_manager,
              ProfileOAuth2TokenService* token_service,
              net::URLRequestContextGetter* request_context,
              leveldb::Env* env_override);
 
   // Called by WorkerObserver.
   void OnPendingFileListUpdated(int item_count);
   void OnFileStatusChanged(const fileapi::FileSystemURL& url,
                            SyncFileStatus file_status,
                            SyncAction sync_action,
                            SyncDirection direction);
   void UpdateServiceState(RemoteServiceState state,
                           const std::string& description);
 
   SyncStatusCallback TrackCallback(const SyncStatusCallback& callback);
 
   scoped_refptr<base::SingleThreadTaskRunner> ui_task_runner_;
   scoped_refptr<base::SequencedTaskRunner> worker_task_runner_;
-  scoped_refptr<base::SequencedTaskRunner> file_task_runner_;
   scoped_refptr<base::SequencedTaskRunner> drive_task_runner_;
 
   const base::FilePath sync_file_system_dir_;
   TaskLogger* task_logger_;
 
   // These external services are not owned by SyncEngine.
   // The owner of the SyncEngine is responsible for their lifetime.
   // I.e. the owner should declare the dependency explicitly by calling
   // KeyedService::DependsOn().
   drive::DriveNotificationManager* notification_manager_;
@@ skipping 29 matching lines @@
   CallbackTracker callback_tracker_;
 
   base::WeakPtrFactory<SyncEngine> weak_ptr_factory_;
   DISALLOW_COPY_AND_ASSIGN(SyncEngine);
 };
 
 }  // namespace drive_backend
 }  // namespace sync_file_system
 
 #endif  // CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_SYNC_ENGINE_H_