Add support for unmixed audio from remote WebRTC remote tracks.

content/renderer/media/webrtc/webrtc_media_stream_adapter.cc

Index: content/renderer/media/webrtc/webrtc_media_stream_adapter.cc
diff --git a/content/renderer/media/webrtc/webrtc_media_stream_adapter.cc b/content/renderer/media/webrtc/webrtc_media_stream_adapter.cc
index 3bc1312cc959fb89f3dab98d968423feb71d8ff4..aece4d0625bed274a76c44b02575c0a8579c2158 100644
--- a/content/renderer/media/webrtc/webrtc_media_stream_adapter.cc
+++ b/content/renderer/media/webrtc/webrtc_media_stream_adapter.cc
@@ -77,7 +77,7 @@ void WebRtcMediaStreamAdapter::CreateAudioTrack(
   DCHECK_EQ(track.source().type(), blink::WebMediaStreamSource::TypeAudio);
   // A media stream is connected to a peer connection, enable the
   // peer connection mode for the sources.
-  MediaStreamTrack* native_track = MediaStreamTrack::GetTrack(track);
+  MediaStreamAudioTrack* native_track = MediaStreamAudioTrack::GetTrack(track);
   if (!native_track || !native_track->is_local_track()) {
     // We don't support connecting remote audio tracks to PeerConnection yet.
     // See issue http://crbug/344303.