Sync the DOMTokenList interface with the spec

Source/core/dom/DOMTokenList.idl

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 // https://dom.spec.whatwg.org/#interface-domtokenlist
+
 [
     SetWrapperReferenceFrom=element,
     WillBeGarbageCollected,
 ] interface DOMTokenList {
     readonly attribute unsigned long length;
     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);
-
+    // FIXME: stringifier should be enumerable.
     [NotEnumerable] stringifier;
     iterable<DOMString>;
 };