Update interfaces/webrtc.idl (#20750)

Source: https://github.com/tidoust/reffy-reports/blob/77c9c30/ed/idl/webrtc.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/624282136
diff --git a/interfaces/webrtc.idl b/interfaces/webrtc.idl
index c21a559..3b2be61 100644
--- a/interfaces/webrtc.idl
+++ b/interfaces/webrtc.idl
@@ -100,7 +100,7 @@
   readonly attribute boolean? canTrickleIceCandidates;
   void restartIce();
   RTCConfiguration getConfiguration();
-  void setConfiguration(RTCConfiguration configuration);
+  void setConfiguration(optional RTCConfiguration configuration = {});
   void close();
   attribute EventHandler onnegotiationneeded;
   attribute EventHandler onicecandidate;