Build WebRTC with data channel only.

webrtc/pc/peerconnection.cc

Index: webrtc/pc/peerconnection.cc
diff --git a/webrtc/pc/peerconnection.cc b/webrtc/pc/peerconnection.cc
index 852dd39de5bc21eed1461340f796d4ef0132e55f..107b997efe032353d8ddbaf2fab011447a16f16d 100644
--- a/webrtc/pc/peerconnection.cc
+++ b/webrtc/pc/peerconnection.cc
@@ -2335,8 +2335,11 @@ void PeerConnection::CreateCall_w() {
   const int kMaxBandwidthBps = 2000000;
 
   webrtc::Call::Config call_config(event_log_.get());
+  if (!factory_->channel_manager()->media_engine()) {
+    return;
+  }
   call_config.audio_state =
-      factory_->channel_manager() ->media_engine()->GetAudioState();
+      factory_->channel_manager()->media_engine()->GetAudioState();
   call_config.bitrate_config.min_bitrate_bps = kMinBandwidthBps;
   call_config.bitrate_config.start_bitrate_bps = kStartBandwidthBps;
   call_config.bitrate_config.max_bitrate_bps = kMaxBandwidthBps;