Do not include task_queue.h in sequenced_task_checker_impl.h

webrtc/base/sequenced_task_checker_impl.h

 /*
  *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #ifndef WEBRTC_BASE_SEQUENCED_TASK_CHECKER_IMPL_H_
 #define WEBRTC_BASE_SEQUENCED_TASK_CHECKER_IMPL_H_
 
-#include "webrtc/base/task_queue.h"
 #include "webrtc/base/thread_checker.h"
 
 namespace rtc {
 
+class TaskQueue;
 // Real implementation of SequencedTaskChecker, for use in debug mode, or
 // for temporary use in release mode.
 //
 // Note: You should almost always use the SequencedTaskChecker class to get the
 // right version for your build configuration.
 class SequencedTaskCheckerImpl {
  public:
   SequencedTaskCheckerImpl();
   ~SequencedTaskCheckerImpl();
 
   bool CalledSequentially() const;
 
   // Changes the task queue or thread that is checked for in IsCurrent.  This
   // may be useful when an object may be created on one task queue / thread and
   // then used exclusively on another thread.
   void Detach();
 
  private:
   CriticalSection lock_;
   ThreadChecker thread_checker_;
   mutable bool attached_;
   mutable TaskQueue* valid_queue_;
 };
 
 }  // namespace rtc
 #endif  // WEBRTC_BASE_SEQUENCED_TASK_CHECKER_IMPL_H_