Add support for unmixed audio from remote WebRTC remote tracks.

content/renderer/media/media_stream_renderer_factory_impl.cc

Index: content/renderer/media/media_stream_renderer_factory_impl.cc
diff --git a/content/renderer/media/media_stream_renderer_factory_impl.cc b/content/renderer/media/media_stream_renderer_factory_impl.cc
index 3163953062809700771066625430c89c483ddc6d..58e7bbcc2e5f3436e73588d4973791cdc3765027 100644
--- a/content/renderer/media/media_stream_renderer_factory_impl.cc
+++ b/content/renderer/media/media_stream_renderer_factory_impl.cc
@@ -141,7 +141,8 @@ MediaStreamRendererFactoryImpl::GetAudioRenderer(
   // and mixes audio from all the tracks that belong to the media stream.
   // For now, we have separate renderers depending on if the first audio track
   // in the stream is local or remote.
-  MediaStreamTrack* audio_track = MediaStreamTrack::GetTrack(audio_tracks[0]);
+  MediaStreamAudioTrack* audio_track = MediaStreamAudioTrack::GetTrack(
+      audio_tracks[0]);
   if (!audio_track) {
     // This can happen if the track was cloned.
     // TODO(tommi, perkj): Fix cloning of tracks to handle extra data too.