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

chrome/browser/sync_file_system/drive_backend/drive_backend_sync_unittest.cc

 // 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.
 
 #include <algorithm>
 #include <stack>
 
 #include "base/file_util.h"
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
@@ skipping 97 matching lines @@
         new drive::DriveUploader(drive_service.get(),
                                  file_task_runner_.get()));
 
     fake_drive_service_helper_.reset(new FakeDriveServiceHelper(
         drive_service.get(), uploader.get(),
         kSyncRootFolderTitle));
 
     remote_sync_service_.reset(new SyncEngine(
         base::ThreadTaskRunnerHandle::Get(),  // ui_task_runner
         worker_task_runner_,
-        file_task_runner_,
         drive_task_runner,
         base_dir_.path(),
         NULL,  // task_logger
         NULL,  // notification_manager
         NULL,  // extension_service
         NULL,  // signin_manager
         NULL,  // token_service
         NULL,  // request_context
         in_memory_env_.get()));
     remote_sync_service_->AddServiceObserver(this);
@@ skipping 1592 matching lines @@
 
   EXPECT_EQ(1u, CountApp());
   EXPECT_EQ(1u, CountLocalFile(app_id));
 
   EXPECT_EQ(2u, CountMetadata());
   EXPECT_EQ(2u, CountTracker());
 }
 
 }  // namespace drive_backend
 }  // namespace sync_file_system