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

chrome/browser/sync_file_system/drive_backend/sync_task_token.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_TASK_TOKEN_H_
 #define CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_SYNC_TASK_TOKEN_H_
 
 #include <stdint.h>
 
 #include <memory>
 
 #include "base/callback.h"
 #include "base/location.h"
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "chrome/browser/sync_file_system/sync_callbacks.h"
 #include "chrome/browser/sync_file_system/task_logger.h"
 
-namespace base {
-class SequencedTaskRunner;
-}
-
 namespace sync_file_system {
 namespace drive_backend {
 
 class SyncTaskManager;
 struct TaskBlocker;
 
 // Represents a running sequence of SyncTasks.  Owned by a callback chain that
 // should run exclusively, and held by SyncTaskManager when no task is running.
 class SyncTaskToken {
  public:
@@ skipping 56 matching lines @@
   std::unique_ptr<TaskLogger::TaskLog> task_log_;
   std::unique_ptr<TaskBlocker> task_blocker_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncTaskToken);
 };
 
 }  // namespace drive_backend
 }  // namespace sync_file_system
 
 #endif  // CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_SYNC_TASK_TOKEN_H_