Fix cases where RunLoop is created without a MessageLoop.

media/base/audio_renderer_mixer_input_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 <memory>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/macros.h"
@@ skipping 106 matching lines @@
   void SwitchCallback(base::RunLoop* loop, OutputDeviceStatus result) {
     SwitchCallbackCalled(result);
     loop->Quit();
   }
 
   AudioRendererMixer* GetInputMixer() { return mixer_input_->mixer_; }
 
  protected:
   virtual ~AudioRendererMixerInputTest() {}
 
+  base::MessageLoop message_loop_;
   AudioParameters audio_parameters_;
   scoped_refptr<MockAudioRendererSink> sinks_[2];
   std::unique_ptr<AudioRendererMixer> mixers_[2];
   scoped_refptr<AudioRendererMixerInput> mixer_input_;
   std::unique_ptr<FakeAudioRenderCallback> fake_callback_;
   std::unique_ptr<AudioBus> audio_bus_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(AudioRendererMixerInputTest);
 };
@@ skipping 259 matching lines @@
       base::Bind(&AudioRendererMixerInputTest::SwitchCallback,
                  base::Unretained(this), &run_loop));
   run_loop.Run();
 
   mixer_input_->Initialize(audio_parameters_, fake_callback_.get());
   mixer_input_->Start();
   mixer_input_->Stop();
 }
 
 }  // namespace media