Make init*Event() arguments nullable where appropriate

third_party/WebKit/Source/core/events/MutationEvent.idl

 /*
  * Copyright (C) 2006 Apple Computer, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 16 matching lines @@
 
     readonly attribute Node?          relatedNode;
     readonly attribute DOMString      prevValue;
     readonly attribute DOMString      newValue;
     readonly attribute DOMString      attrName;
     readonly attribute unsigned short attrChange;
     // TODO(philipj): None of the initMutationEvent() arguments should be optional.
     [Measure, LegacyInterfaceTypeChecking] void initMutationEvent([Default=Undefined] optional DOMString type,
                                                                   [Default=Undefined] optional boolean bubbles,
                                                                   [Default=Undefined] optional boolean cancelable,
-                                                                  [Default=Undefined] optional Node relatedNode,
+                                                                  [Default=Undefined] optional Node? relatedNode,
                                                                   [Default=Undefined] optional DOMString prevValue,
                                                                   [Default=Undefined] optional DOMString newValue,
                                                                   [Default=Undefined] optional DOMString attrName,
                                                                   [Default=Undefined] optional unsigned short attrChange);
 };