Add support for unmixed audio from remote WebRTC remote tracks.

content/renderer/media/webrtc_audio_renderer.cc

Index: content/renderer/media/webrtc_audio_renderer.cc
diff --git a/content/renderer/media/webrtc_audio_renderer.cc b/content/renderer/media/webrtc_audio_renderer.cc
index f61147e29c5f6435963c650f2fffe65b10846ef6..cefc85d20014410064c31a422b2a0b9510de96f8 100644
--- a/content/renderer/media/webrtc_audio_renderer.cc
+++ b/content/renderer/media/webrtc_audio_renderer.cc
@@ -554,7 +554,7 @@ void WebRtcAudioRenderer::OnPlayStateChanged(
   media_stream.audioTracks(web_tracks);
 
   for (const blink::WebMediaStreamTrack& web_track : web_tracks) {
-    MediaStreamTrack* track = MediaStreamTrack::GetTrack(web_track);
+    MediaStreamAudioTrack* track = MediaStreamAudioTrack::GetTrack(web_track);
     // WebRtcAudioRenderer can only render audio tracks received from a remote
     // peer. Since the actual MediaStream is mutable from JavaScript, we need
     // to make sure |web_track| is actually a remote track.