Build WebRTC with data channel only.

webrtc/media/base/fakemediaengine.h

Index: webrtc/media/base/fakemediaengine.h
diff --git a/webrtc/media/base/fakemediaengine.h b/webrtc/media/base/fakemediaengine.h
index c0aacfce5c19e6535c169096d859d3837062b3ee..f2dc290e54e1759815b71f235240466fbddca061 100644
--- a/webrtc/media/base/fakemediaengine.h
+++ b/webrtc/media/base/fakemediaengine.h
@@ -424,6 +424,10 @@ class FakeVoiceMediaChannel : public RtpHelper<VoiceMediaChannel> {
     sink_ = std::move(sink);
   }
 
+  virtual std::vector<webrtc::RtpSource> GetSources(uint32_t ssrc) const {
+    return std::vector<webrtc::RtpSource>();
+  }
+
  private:
   class VoiceChannelAudioSink : public AudioSource::Sink {
    public: