Build WebRTC with data channel only.

webrtc/BUILD.gn

Index: webrtc/BUILD.gn
diff --git a/webrtc/BUILD.gn b/webrtc/BUILD.gn
index 7649f46b8ae8b97538f2064564c4fe7c9994dc9d..5a81e8b58e7fb7e5c9fa40e600f5afa6c612e5dd 100644
--- a/webrtc/BUILD.gn
+++ b/webrtc/BUILD.gn
@@ -126,6 +126,10 @@ config("common_config") {
     defines += [ "HAVE_SCTP" ]
   }
 
+  if (rtc_enable_media) {
+    defines += [ "HAVE_MEDIA" ]
+  }
+
   if (rtc_enable_external_auth) {
     defines += [ "ENABLE_EXTERNAL_AUTH" ]
   }