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

chrome/browser/sync_file_system/drive_backend/sync_engine_context.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_CONTEXT_H_
 #define CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_SYNC_ENGINE_CONTEXT_H_
 
 #include <memory>
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "base/sequence_checker.h"
 
 namespace base {
 class SequencedTaskRunner;
-class SequencedWorkerPool;
 class SingleThreadTaskRunner;
 }
 
 namespace drive {
 class DriveServiceInterface;
 class DriveUploaderInterface;
 }
 
 namespace sync_file_system {
 
@@ skipping 47 matching lines @@
 
   base::SequenceChecker sequence_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncEngineContext);
 };
 
 }  // namespace drive_backend
 }  // namespace sync_file_system
 
 #endif  // CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_SYNC_ENGINE_CONTEXT_H_