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..4a0165781918bf2c7eec902236123d69d94eb688 100644
--- a/media/base/audio_renderer_mixer_input.h
+++ b/media/base/audio_renderer_mixer_input.h
@@ -9,6 +9,7 @@
 
 #include "base/callback.h"
 #include "base/macros.h"
+#include "base/synchronization/lock.h"
 #include "media/base/audio_converter.h"
 #include "media/base/audio_renderer_sink.h"
 #include "media/base/output_device.h"
@@ -69,13 +70,30 @@ class MEDIA_EXPORT AudioRendererMixerInput
  private:
   friend class AudioRendererMixerInputTest;
 
+  // AudioConverter::InputCallback implementation.
+  double ProvideInput(AudioBus* audio_bus,
+                      base::TimeDelta buffer_delay) override;
+
+  // The implementation for these methods is split out to avoid re-entrant
+  // locking when called by multiple internal callers. Re-entrant locking is
+  // not supported with base::Lock.
+  void Stop_Locked();
+  void Play_Locked();
+  void Pause_Locked();
+  void SwitchOutputDevice_Locked(const std::string& device_id,
+                                 const url::Origin& security_origin,
+                                 const SwitchOutputDeviceCB& callback);
+
+  // This class is used by both media thread (e.g. SetVolume) and audio device
+  // thread (e.g. ProvideInput).
+  base::Lock lock_;
+
+  // ---------------[ All variables below protected by |lock_| ]---------------
+
   bool started_;
   bool playing_;
   double volume_;
 
-  // AudioConverter::InputCallback implementation.
-  double ProvideInput(AudioBus* audio_bus,
-                      base::TimeDelta buffer_delay) override;
 
   // Callbacks provided during construction which allow AudioRendererMixerInput
   // to retrieve a mixer during Initialize() and notify when it's done with it.