Add audio support in WebrtcConnectionToHost, audio unittest

remoting/protocol/webrtc_video_renderer_adapter.cc

Index: remoting/protocol/webrtc_video_renderer_adapter.cc
diff --git a/remoting/protocol/webrtc_video_renderer_adapter.cc b/remoting/protocol/webrtc_video_renderer_adapter.cc
index cb6a360d0025c6fc5f1784865dbae3944fe37e85..c0cddbcf78c094232e1cec1f28e86a899b39fd40 100644
--- a/remoting/protocol/webrtc_video_renderer_adapter.cc
+++ b/remoting/protocol/webrtc_video_renderer_adapter.cc
@@ -78,10 +78,9 @@ void WebrtcVideoRendererAdapter::SetMediaStream(
   media_stream_ = std::move(media_stream);
 
   webrtc::VideoTrackVector video_tracks = media_stream_->GetVideoTracks();
-  if (video_tracks.empty()) {
-    LOG(ERROR) << "Received media stream with no video tracks.";
-    return;
-  }
+
+  // Caller must verify that the media stream contains video tracks.
+  DCHECK(!video_tracks.empty());
 
   if (video_tracks.size() > 1U) {
     LOG(WARNING) << "Received media stream with multiple video tracks.";