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

webrtc/common_video/i420_buffer_pool.cc

 /*
  *  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.
  */
 
 #include "webrtc/common_video/include/i420_buffer_pool.h"
 
 #include "webrtc/base/checks.h"
 
 namespace webrtc {
 
 I420BufferPool::I420BufferPool(bool zero_initialize)
     : zero_initialize_(zero_initialize) {
-  thread_checker_.DetachFromThread();
+  sequenced_checker_.Detach();
 }
 
 void I420BufferPool::Release() {
-  thread_checker_.DetachFromThread();
+  sequenced_checker_.Detach();
   buffers_.clear();
 }
 
 rtc::scoped_refptr<I420Buffer> I420BufferPool::CreateBuffer(int width,
                                                             int height) {
-  RTC_DCHECK(thread_checker_.CalledOnValidThread());
+  RTC_DCHECK(sequenced_checker_.CalledSequentially());
   // Release buffers with wrong resolution.
   for (auto it = buffers_.begin(); it != buffers_.end();) {
     if ((*it)->width() != width || (*it)->height() != height)
       it = buffers_.erase(it);
     else
       ++it;
   }
   // Look for a free buffer.
   for (const rtc::scoped_refptr<PooledI420Buffer>& buffer : buffers_) {
     // If the buffer is in use, the ref count will be >= 2, one from the list we
     // are looping over and one from the application. If the ref count is 1,
     // then the list we are looping over holds the only reference and it's safe
     // to reuse.
     if (buffer->HasOneRef())
       return buffer;
   }
   // Allocate new buffer.
   rtc::scoped_refptr<PooledI420Buffer> buffer =
       new PooledI420Buffer(width, height);
   if (zero_initialize_)
     buffer->InitializeData();
   buffers_.push_back(buffer);
   return buffer;
 }
 
 }  // namespace webrtc