IDL: Support optional argument default value syntax

Source/core/html/HTMLSelectElement.idl

 /*
  * Copyright (C) 2006, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  * Copyright (C) 2013 Samsung Electronics. All rights reserved.
  *
  * 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.
  *
@@ skipping 20 matching lines @@
     attribute long size;
 
     readonly attribute DOMString type;
 
     readonly attribute HTMLOptionsCollection options;
     [RaisesException=Setter] attribute unsigned long length;
 
     getter Element item(unsigned long index);
     Element namedItem([Default=Undefined] optional DOMString name);
     // FIXME: should be union type http://crbug.com/240176
-    [RaisesException, TypeChecking=Interface] void add(HTMLElement element, [Default=Null] optional HTMLElement? before);
+    [RaisesException, TypeChecking=Interface] void add(HTMLElement element, optional HTMLElement? before = null);
     [ImplementedAs=addBeforeOptionAtIndex, RaisesException, TypeChecking=Interface] void add(HTMLElement element, long before);
     [RaisesException] void remove(); // ChildNode overload
     void remove(long index);
     [RaisesException, TypeChecking=Interface|Nullable] setter HTMLOptionElement (unsigned long index, HTMLOptionElement? value);
 
     readonly attribute HTMLCollection selectedOptions;
              attribute long selectedIndex;
              attribute DOMString value;
 
     readonly attribute boolean willValidate;
     readonly attribute ValidityState validity;
     readonly attribute DOMString validationMessage;
     boolean checkValidity();
     void setCustomValidity([TreatNullAs=NullString, TreatUndefinedAs=NullString] DOMString error);
 
     readonly attribute NodeList labels;
 };