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

Source/core/css/MediaList.idl

 /*
  * Copyright (C) 2006, 2007 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 14 matching lines @@
 
 // http://dev.w3.org/csswg/cssom/#the-medialist-interface
 
 // TODO(philipj): MediaList should be an [ArrayClass].
 [
     WillBeGarbageCollected
 ] interface MediaList {
     // TODO(philipj): [TreatNullAs=EmptyString] stringifier attribute DOMString mediaText;
     attribute DOMString? mediaText;
     readonly attribute unsigned long length;
-    getter DOMString? item(unsigned long index);
+    [Measure] getter DOMString? item(unsigned long index);
     // TODO(philipj): appendMedium() and deleteMedium() should never throw.
     [RaisesException] void appendMedium(DOMString medium);
     [RaisesException] void deleteMedium(DOMString medium);
 };