Add missing IWYU message_loop.h includes.

media/filters/ffmpeg_demuxer_unittest.cc

 // Copyright (c) 2012 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.
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
 #include <deque>
 #include <string>
 
 #include "base/bind.h"
 #include "base/files/file_path.h"
 #include "base/location.h"
 #include "base/logging.h"
 #include "base/macros.h"
+#include "base/message_loop/message_loop.h"
 #include "base/path_service.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/test/mock_callback.h"
 #include "base/test/scoped_task_scheduler.h"
 #include "base/threading/thread.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "media/base/decrypt_config.h"
 #include "media/base/media_log.h"
 #include "media/base/media_tracks.h"
@@ skipping 1626 matching lines @@
   // ensuring that status changes occur while an async read is pending.
   audio_stream->FlushBuffers();
   audio_stream->Read(base::Bind(&media::OnReadDone_ExpectEos));
   CheckStreamStatusNotifications(demuxer_.get(), audio_stream);
   video_stream->FlushBuffers();
   video_stream->Read(base::Bind(&media::OnReadDone_ExpectEos));
   CheckStreamStatusNotifications(demuxer_.get(), video_stream);
 }
 
 }  // namespace media