Add support for unmixed audio from remote WebRTC remote tracks.

content/renderer/media/rtc_peer_connection_handler.cc

Index: content/renderer/media/rtc_peer_connection_handler.cc
diff --git a/content/renderer/media/rtc_peer_connection_handler.cc b/content/renderer/media/rtc_peer_connection_handler.cc
index f7393c1d3b63c953177cbe1961538ec62ef55f5a..f700566fa2ada657c267edd5fcd5b74296c7fed0 100644
--- a/content/renderer/media/rtc_peer_connection_handler.cc
+++ b/content/renderer/media/rtc_peer_connection_handler.cc
@@ -19,6 +19,7 @@
 #include "base/thread_task_runner_handle.h"
 #include "base/trace_event/trace_event.h"
 #include "content/public/common/content_switches.h"
+#include "content/renderer/media/media_stream_audio_track.h"
 #include "content/renderer/media/media_stream_track.h"
 #include "content/renderer/media/peer_connection_tracker.h"
 #include "content/renderer/media/remote_media_stream_impl.h"
@@ -1361,7 +1362,7 @@ blink::WebRTCDTMFSenderHandler* RTCPeerConnectionHandler::createDTMFSender(
   TRACE_EVENT0("webrtc", "RTCPeerConnectionHandler::createDTMFSender");
   DVLOG(1) << "createDTMFSender.";
 
-  MediaStreamTrack* native_track = MediaStreamTrack::GetTrack(track);
+  MediaStreamAudioTrack* native_track = MediaStreamAudioTrack::GetTrack(track);
   if (!native_track || !native_track->is_local_track() ||
       track.source().type() != blink::WebMediaStreamSource::TypeAudio) {
     DLOG(ERROR) << "The DTMF sender requires a local audio track.";