Reland "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 824d6b255846941ed70422d3c395770015fb4a13..bf67f23641d3d7fc792cdd1a0f85a057c1b6b00a 100644
--- a/content/renderer/media/webrtc_audio_device_impl.h
+++ b/content/renderer/media/webrtc_audio_device_impl.h
@@ -240,9 +240,15 @@ 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() {}
   };