Replace [TreatReturnedNullStringAs=Null] with nullable return type

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 13 matching lines @@
  */
 
 // Introduced in DOM Level 2:
 [
     WillBeGarbageCollected
 ] interface MediaList {
 
              attribute DOMString? mediaText;
     readonly attribute unsigned long length;
 
-    [TreatReturnedNullStringAs=Null] getter DOMString item(unsigned long index);
+    getter DOMString? item(unsigned long index);
     [RaisesException] void deleteMedium(DOMString oldMedium);
     [RaisesException] void appendMedium(DOMString newMedium);
 
 };