IDL: Support optional argument default value syntax

Source/core/html/HTMLInputElement.idl

 /*
  * Copyright (C) 2006, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  * Copyright (C) 2012 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 67 matching lines @@
 
     void select();
     [RaisesException, ImplementedAs=selectionStartForBinding] attribute long selectionStart;
     [RaisesException, ImplementedAs=selectionEndForBinding] attribute long selectionEnd;
     [RaisesException, ImplementedAs=selectionDirectionForBinding] attribute DOMString selectionDirection;
 
     [RaisesException] void setRangeText(DOMString replacement);
     [RaisesException] void setRangeText(DOMString replacement,
                         unsigned long start,
                         unsigned long end,
-                        [Default=NullString] optional DOMString selectionMode);
+                        optional DOMString selectionMode = null);
 
     [RaisesException, ImplementedAs=setSelectionRangeForBinding]
     void setSelectionRange([Default=Undefined] optional long start,
                            [Default=Undefined] optional long end,
                            optional DOMString direction);
 
     // Non-standard attributes
     [Reflect, MeasureAs=PrefixedDirectoryAttribute] attribute boolean webkitdirectory;
     [Reflect, MeasureAs=IncrementalAttribute] attribute boolean incremental;
 
     // See http://www.w3.org/TR/html-media-capture/
     [Conditional=MEDIA_CAPTURE, Reflect] attribute boolean capture;
 };