Revert of Remove header dependencies from sequence_checker.h to sequenced_worker_pool.h

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 <memory>
 #include <set>
 #include <string>
 
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "base/observer_list.h"
 #include "chrome/browser/sync_file_system/drive_backend/callback_tracker.h"
 #include "chrome/browser/sync_file_system/local_change_processor.h"
 #include "chrome/browser/sync_file_system/remote_file_sync_service.h"
 #include "chrome/browser/sync_file_system/sync_action.h"
 #include "chrome/browser/sync_file_system/sync_direction.h"
 #include "components/drive/drive_notification_observer.h"
 #include "components/drive/service/drive_service_interface.h"
 #include "components/signin/core/browser/signin_manager_base.h"
 #include "net/base/network_change_notifier.h"
 
 class ExtensionServiceInterface;
 class OAuth2TokenService;
 
 namespace base {
 class SequencedTaskRunner;
-class SequencedWorkerPool;
 }
 
 namespace drive {
 class DriveServiceInterface;
 class DriveNotificationManager;
 class DriveUploaderInterface;
 }
 
 namespace leveldb {
 class Env;
@@ skipping 190 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_