Avoid hitting the thread check when WebRtcAudioRenderer is going away.

content/renderer/media/webrtc_audio_device_impl.h

Index: content/renderer/media/webrtc_audio_device_impl.h
diff --git a/content/renderer/media/webrtc_audio_device_impl.h b/content/renderer/media/webrtc_audio_device_impl.h
index 7354002dcaa6c79129ecf24e5517b2917bc9e14a..80b0cd5dee305768e22b7b77573be91adc99c292 100644
--- a/content/renderer/media/webrtc_audio_device_impl.h
+++ b/content/renderer/media/webrtc_audio_device_impl.h
@@ -239,9 +239,15 @@ class WebRtcPlayoutDataSource {
   class Sink {
    public:
     // Callback to get the playout data.
+    // Called on the render audio thread.
     virtual void OnPlayoutData(media::AudioBus* audio_bus,
                                int sample_rate,
                                int audio_delay_milliseconds) = 0;
+
+    // Callback to notify the sink that the source has changed.
+    // Called on the main render thread.
+    virtual void OnPlayoutDataSourceChanged() = 0;
+
    protected:
     virtual ~Sink() {}
   };