Make more [TreatNullAs=NullString] DOMString arguments nullable

Source/modules/mediastream/RTCPeerConnection.idl

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 49 matching lines @@
 
     sequence<MediaStream> getLocalStreams();
     sequence<MediaStream> getRemoteStreams();
     MediaStream getStreamById(DOMString streamId);
 
     [TypeChecking=Interface, RaisesException] void addStream(MediaStream? stream, optional Dictionary mediaConstraints);
     [TypeChecking=Interface, RaisesException] void removeStream(MediaStream? stream);
 
     void getStats(RTCStatsCallback successCallback, [Default=Undefined] optional MediaStreamTrack selector);
 
-    [RaisesException] RTCDataChannel createDataChannel([TreatNullAs=NullString, TreatUndefinedAs=NullString] DOMString label, optional Dictionary options);
+    [RaisesException] RTCDataChannel createDataChannel([TreatUndefinedAs=NullString] DOMString? label, optional Dictionary options);
 
     [RaisesException] RTCDTMFSender createDTMFSender(MediaStreamTrack track);
 
     [RaisesException] void close();
 
     attribute EventHandler onnegotiationneeded;
     attribute EventHandler onicecandidate;
     attribute EventHandler onsignalingstatechange;
     attribute EventHandler onaddstream;
     attribute EventHandler onremovestream;
     attribute EventHandler oniceconnectionstatechange;
     attribute EventHandler ondatachannel;
 };