Replace [TreatReturnedNullStringAs=Null] with nullable return type

Source/core/dom/DOMTokenList.idl

 /*
  * Copyright (C) 2010, Google 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 10 matching lines @@
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 // https://dom.spec.whatwg.org/#interface-domtokenlist
 [
     SetWrapperReferenceFrom=element,
     WillBeGarbageCollected,
 ] interface DOMTokenList {
     readonly attribute unsigned long length;
-    [TreatReturnedNullStringAs=Null] getter DOMString item(unsigned long index);
+    getter DOMString? item(unsigned long index);
     [RaisesException] boolean contains(DOMString token);
     [RaisesException, CustomElementCallbacks] void add(DOMString... tokens);
     [RaisesException, CustomElementCallbacks] void remove(DOMString... tokens);
     [RaisesException, CustomElementCallbacks] boolean toggle(DOMString token, optional boolean force);
 
     [NotEnumerable] stringifier;
     iterable<DOMString>;
 };