Make second argument to CSSStyleSheet.insertRule() optional again

Source/core/css/CSSStyleSheet.idl

 /*
  * Copyright (C) 2006, 2007, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  *
  * 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
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  */
 
 // Introduced in DOM Level 2:
 [
     GenerateIsReachable=ownerNode
 ] interface CSSStyleSheet : StyleSheet {
     readonly attribute CSSRule          ownerRule;
     readonly attribute CSSRuleList      cssRules;
 
-    [RaisesException] unsigned long insertRule(DOMString rule, unsigned long index);
+    [RaisesException] unsigned long insertRule(DOMString rule, optional unsigned long index);
     [RaisesException] void deleteRule(unsigned long index);
 
     // IE Extensions
     readonly attribute CSSRuleList      rules;
 
     [RaisesException] long addRule([Default=Undefined] optional DOMString selector,
                  [Default=Undefined] optional DOMString style,
                  optional unsigned long index);
     [RaisesException] void removeRule([Default=Undefined] optional unsigned long index);
 };