DOMTokenList.contains() should not throw.

third_party/WebKit/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 12 matching lines @@
  * DAMAGE.
  */
 
 // https://dom.spec.whatwg.org/#interface-domtokenlist
 
 [
     DependentLifetime,
 ] interface DOMTokenList {
     readonly attribute unsigned long length;
     getter DOMString? item(unsigned long index);
-    [RaisesException] boolean contains(DOMString token);
+    boolean contains(DOMString token);
     [RaisesException, CEReactions, CustomElementCallbacks] void add(DOMString... tokens);
     [RaisesException, CEReactions, CustomElementCallbacks] void remove(DOMString... tokens);
     [RaisesException, CEReactions, CustomElementCallbacks] boolean toggle(DOMString token, optional boolean force);
     [RaisesException, CustomElementCallbacks] boolean supports(DOMString token);
     [CEReactions] attribute DOMString value;
     stringifier;
     iterable<DOMString>;
 };