Remove dangling PlayingState pointers in WebRtcAudioRenderer.

content/renderer/media/webrtc_audio_renderer.h

Index: content/renderer/media/webrtc_audio_renderer.h
diff --git a/content/renderer/media/webrtc_audio_renderer.h b/content/renderer/media/webrtc_audio_renderer.h
index 1da6863cb2062447c10119f94712a229b24c9e70..f6e1a4614a8cb07e8d57f47a09740543adeeb890 100644
--- a/content/renderer/media/webrtc_audio_renderer.h
+++ b/content/renderer/media/webrtc_audio_renderer.h
@@ -8,6 +8,7 @@
 #include <stdint.h>
 
 #include <map>
+#include <memory>
 #include <string>
 #include <vector>
 
@@ -193,6 +194,9 @@ class CONTENT_EXPORT WebRtcAudioRenderer
   void OnPlayStateChanged(const blink::WebMediaStream& media_stream,
                           PlayingState* state);
 
+  // Called when |state| is about to be destructed.
+  void OnPlayStateRemoved(PlayingState* state);
+
   // Updates |sink_params_| and |audio_fifo_| based on |sink_|, and initializes
   // |sink_|.
   void PrepareSink();