Reland of Switch to use SequencedTaskChecker instead of ThreadChecker where needed.

webrtc/common_video/include/i420_buffer_pool.h

 /*
  *  Copyright (c) 2015 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_COMMON_VIDEO_INCLUDE_I420_BUFFER_POOL_H_
 #define WEBRTC_COMMON_VIDEO_INCLUDE_I420_BUFFER_POOL_H_
 
 #include <list>
 
-#include "webrtc/base/thread_checker.h"
+#include "webrtc/base/sequenced_task_checker.h"
 #include "webrtc/common_video/include/video_frame_buffer.h"
 
 namespace webrtc {
 
 // Simple buffer pool to avoid unnecessary allocations of I420Buffer objects.
 // The pool manages the memory of the I420Buffer returned from CreateBuffer.
 // When the I420Buffer is destructed, the memory is returned to the pool for use
 // by subsequent calls to CreateBuffer. If the resolution passed to CreateBuffer
 // changes, old buffers will be purged from the pool.
 class I420BufferPool {
  public:
   I420BufferPool() : I420BufferPool(false) {}
   explicit I420BufferPool(bool zero_initialize);
 
   // Returns a buffer from the pool, or creates a new buffer if no suitable
   // buffer exists in the pool.
   rtc::scoped_refptr<I420Buffer> CreateBuffer(int width, int height);
   // Clears buffers_ and detaches the thread checker so that it can be reused
   // later from another thread.
   void Release();
 
  private:
   // Explicitly use a RefCountedObject to get access to HasOneRef,
   // needed by the pool to check exclusive access.
   using PooledI420Buffer = rtc::RefCountedObject<I420Buffer>;
 
-  rtc::ThreadChecker thread_checker_;
+  rtc::SequencedTaskChecker sequenced_checker_;
   std::list<rtc::scoped_refptr<PooledI420Buffer>> buffers_;
   // If true, newly allocated buffers are zero-initialized. Note that recycled
   // buffers are not zero'd before reuse. This is required of buffers used by
   // FFmpeg according to http://crbug.com/390941, which only requires it for the
   // initial allocation (as shown by FFmpeg's own buffer allocation code). It
   // has to do with "Use-of-uninitialized-value" on "Linux_msan_chrome".
   bool zero_initialize_;
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_COMMON_VIDEO_INCLUDE_I420_BUFFER_POOL_H_