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

chrome/browser/sync_file_system/drive_backend/sync_engine_initializer.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_initializer.h"
 
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/logging.h"
 #include "chrome/browser/drive/drive_api_service.h"
@@ skipping 69 matching lines @@
   // The metadata seems to have been already initialized. Just return with OK.
   if (sync_context_->GetMetadataDatabase()) {
     util::Log(logging::LOG_VERBOSE, FROM_HERE,
               "[Initialize] Already initialized.");
     SyncTaskManager::NotifyTaskDone(token.Pass(), SYNC_STATUS_OK);
     return;
   }
 
   MetadataDatabase::Create(
       sync_context_->GetWorkerTaskRunner(),
-      sync_context_->GetFileTaskRunner(),
       database_path_,
       env_override_,
       base::Bind(&SyncEngineInitializer::DidCreateMetadataDatabase,
                  weak_ptr_factory_.GetWeakPtr(), base::Passed(&token)));
 }
 
 scoped_ptr<MetadataDatabase> SyncEngineInitializer::PassMetadataDatabase() {
   return metadata_database_.Pass();
 }
 
@@ skipping 266 matching lines @@
     return;
   }
 
   util::Log(logging::LOG_VERBOSE, FROM_HERE,
             "[Initialize] Completed successfully.");
   SyncTaskManager::NotifyTaskDone(token.Pass(), SYNC_STATUS_OK);
 }
 
 }  // namespace drive_backend
 }  // namespace sync_file_system