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

Source/core/css/CSSRuleList.idl

 /*
  * Copyright (C) 2006, 2007, 2009 Apple 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 12 matching lines @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 // http://dev.w3.org/csswg/cssom/#the-cssrulelist-interface
 
 // TODO(philipj): CSSRuleList should be an [ArrayClass].
 [
     DependentLifetime,
     WillBeGarbageCollected,
 ] interface CSSRuleList {
-    getter CSSRule? item(unsigned long index);
+    [Measure] getter CSSRule? item(unsigned long index);
     readonly attribute unsigned long length;
 };