Add UseCounters for CSS-related *List interfaces with item() methods

Source/core/css/StyleSheetList.idl

 /*
  * Copyright (C) 2006, 2007, 2009 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.
  */
 
 // http://dev.w3.org/csswg/cssom/#the-stylesheetlist-interface
 
 // TODO(philipj): StyleSheetList should be an [ArrayClass].
 [
     SetWrapperReferenceFrom=document,
     WillBeGarbageCollected
 ] interface StyleSheetList {
-    getter StyleSheet? item(unsigned long index);
+    [Measure] getter StyleSheet? item(unsigned long index);
     readonly attribute unsigned long length;
 
     // Non-standard APIs
     [NotEnumerable] getter CSSStyleSheet (DOMString name);
 };