Build WebRTC with data channel only.

webrtc/pc/channel.cc

Index: webrtc/pc/channel.cc
diff --git a/webrtc/pc/channel.cc b/webrtc/pc/channel.cc
index f5428a43effac7a93fce554e11946c220946337c..8b2edfc16f6e78f18d7297e4b3a11f869f87aa17 100644
--- a/webrtc/pc/channel.cc
+++ b/webrtc/pc/channel.cc
@@ -1607,10 +1607,14 @@ bool VoiceChannel::GetStats(VoiceMediaInfo* stats) {
 }
 
 std::vector<webrtc::RtpSource> VoiceChannel::GetSources(uint32_t ssrc) const {
+#ifdef HAVE_MEDIA
   return worker_thread()->Invoke<std::vector<webrtc::RtpSource>>(
       RTC_FROM_HERE,
       Bind(&WebRtcVoiceMediaChannel::GetSources,
            static_cast<WebRtcVoiceMediaChannel*>(media_channel()), ssrc));
+#else
+  return std::vector<webrtc::RtpSource>();
+#endif
 }
 
 void VoiceChannel::StartMediaMonitor(int cms) {