Use ScopedTaskEnvironment instead of MessageLoop in media unit tests.

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 cec6312f9750d7d2652e79a12a3dee4d5f980ad1..8f6a239b4cb96354eb97f163e2966e7d6676b178 100644
--- a/media/base/audio_renderer_mixer_input_unittest.cc
+++ b/media/base/audio_renderer_mixer_input_unittest.cc
@@ -9,6 +9,7 @@
 #include "base/bind_helpers.h"
 #include "base/macros.h"
 #include "base/run_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "media/base/audio_latency.h"
 #include "media/base/audio_renderer_mixer.h"
 #include "media/base/audio_renderer_mixer_input.h"
@@ -124,7 +125,7 @@ class AudioRendererMixerInputTest : public testing::Test,
  protected:
   virtual ~AudioRendererMixerInputTest() {}
 
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   AudioParameters audio_parameters_;
   scoped_refptr<MockAudioRendererSink> sinks_[2];
   std::unique_ptr<AudioRendererMixer> mixers_[2];