Add missing IWYU message_loop.h includes.

chromecast/media/audio/cast_audio_output_stream_unittest.cc

 // Copyright 2015 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 "chromecast/media/audio/cast_audio_output_stream.h"
 
 #include <stdint.h>
 
 #include <memory>
 #include <utility>
 
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/time/time.h"
 #include "chromecast/base/metrics/cast_metrics_test_helper.h"
 #include "chromecast/media/audio/cast_audio_manager.h"
 #include "chromecast/public/media/cast_decoder_buffer.h"
 #include "chromecast/public/media/media_pipeline_backend.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 using ::testing::Invoke;
@@ skipping 533 matching lines @@
 
   stream->Start(source_callback.get());
   RunMessageLoopFor(2);
   stream->Stop();
   stream->Close();
 }
 
 }  // namespace
 }  // namespace media
 }  // namespace chromecast