Fix cases where RunLoop is created without a MessageLoop.

media/base/audio_renderer_mixer_input_unittest.cc

Index: media/base/audio_renderer_mixer_input_unittest.cc
diff --git a/media/base/audio_renderer_mixer_input_unittest.cc b/media/base/audio_renderer_mixer_input_unittest.cc
index c3c37a8347c19043b5198470fb936c4ff903d941..1cfd8d6840d28de08af04939b83f31944e7b5de6 100644
--- a/media/base/audio_renderer_mixer_input_unittest.cc
+++ b/media/base/audio_renderer_mixer_input_unittest.cc
@@ -124,6 +124,7 @@ class AudioRendererMixerInputTest : public testing::Test,
  protected:
   virtual ~AudioRendererMixerInputTest() {}
 
+  base::MessageLoop message_loop_;
   AudioParameters audio_parameters_;
   scoped_refptr<MockAudioRendererSink> sinks_[2];
   std::unique_ptr<AudioRendererMixer> mixers_[2];