Make the setCustomValidity() error argument not nullable

Source/core/html/HTMLKeygenElement.idl

 /*
  * Copyright (C) 2011 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 27 matching lines @@
     [Reflect, ReflectOnly="rsa", ReflectMissing="rsa"] attribute DOMString keytype;
     [Reflect] attribute DOMString name;
 
     readonly attribute DOMString type;
 
     readonly attribute boolean willValidate;
     readonly attribute ValidityState validity;
     readonly attribute DOMString validationMessage;
     boolean checkValidity();
     boolean reportValidity();
-    // FIXME: The error argument should not be nullable.
-    void setCustomValidity([TreatUndefinedAs=NullString] DOMString? error);
+    void setCustomValidity(DOMString error);
 
     readonly attribute NodeList labels;
 };