IDL: Support optional argument default value syntax

Source/core/frame/Window.idl

 /*
  * Copyright (C) 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2011 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 51 matching lines @@
     [Custom] Window open(DOMString url,
                             DOMString name,
                             optional DOMString options);
 
     [RuntimeEnabled=ShowModalDialog, Custom] any showModalDialog(DOMString url,
                                        optional any dialogArgs,
                                        optional DOMString featureArgs);
 
     void alert();
     void alert(DOMString message);
-    boolean confirm([Default=NullString] optional DOMString message);
-    [TreatReturnedNullStringAs=Null] DOMString prompt([Default=NullString] optional DOMString message,
-                                                      [Default=NullString] optional DOMString defaultValue);
+    boolean confirm(optional DOMString message = null);
+    [TreatReturnedNullStringAs=Null] DOMString prompt(optional DOMString message = null,
+                                                      optional DOMString defaultValue = null);
 
     boolean find([Default=Undefined] optional DOMString string,
                  [Default=Undefined] optional boolean caseSensitive,
                  [Default=Undefined] optional boolean backwards,
                  [Default=Undefined] optional boolean wrap,
                  [Default=Undefined] optional boolean wholeWord,
                  [Default=Undefined] optional boolean searchInFrames,
                  [Default=Undefined] optional boolean showDialog);
 
     [Replaceable, MeasureAs=WindowOffscreenBuffering] readonly attribute boolean offscreenBuffering;
@@ skipping 126 matching lines @@
     [DoNotCheckSignature, DoNotCheckSecurity, Custom, NotEnumerable] DOMString toString();
 
     [NotEnumerable] getter Window (unsigned long index);
     [Custom, NotEnumerable] getter Window (DOMString name);
 };
 
 Window implements GlobalEventHandlers;
 Window implements WindowBase64;
 Window implements WindowEventHandlers;
 Window implements WindowTimers;