Add lock to fix race in AudioRendererMixerInput.

media/base/audio_renderer_mixer_input.h

Index: media/base/audio_renderer_mixer_input.h
diff --git a/media/base/audio_renderer_mixer_input.h b/media/base/audio_renderer_mixer_input.h
index 34f37c5ee00a3c0d0803ef4f807992942bbf8401..6ae5a4bcd76be784f9ac3e21aaa8415564456313 100644
--- a/media/base/audio_renderer_mixer_input.h
+++ b/media/base/audio_renderer_mixer_input.h
@@ -9,6 +9,8 @@
 
 #include "base/callback.h"
 #include "base/macros.h"
+#include "base/synchronization/lock.h"
+#include "base/threading/thread_checker.h"
 #include "media/base/audio_converter.h"
 #include "media/base/audio_renderer_sink.h"
 #include "media/base/output_device.h"
@@ -69,6 +71,14 @@ class MEDIA_EXPORT AudioRendererMixerInput
  private:
   friend class AudioRendererMixerInputTest;
 
+  // Used to DCHECK that control methods (Start/Stop/Switch...) are called from
+  // the same thread.
+  base::ThreadChecker thread_checker_;
+
+  // Protect |volume_|, accessed by separate threads in ProvideInput() and
+  // SetVolume().
+  base::Lock volume_lock_;
+
   bool started_;
   bool playing_;
   double volume_;