Make the setCustomValidity() error argument not nullable

Source/core/html/HTMLObjectElement.idl

 /*
  * Copyright (C) 2006, 2007, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 24 matching lines @@
     [Reflect] attribute DOMString height;
     [CheckSecurity=Node] readonly attribute Document? contentDocument;
     // TODO(philipj): readonly attribute WindowProxy? contentWindow;
     [CheckSecurity=Node, RaisesException] Document? getSVGDocument();
 
     readonly attribute boolean willValidate;
     readonly attribute ValidityState validity;
     readonly attribute DOMString validationMessage;
     boolean checkValidity();
     boolean reportValidity();
-    // TODO(philipj):  The error argument should not be nullable.
-    void setCustomValidity([TreatUndefinedAs=NullString] DOMString? error);
+    void setCustomValidity(DOMString error);
 
     // TODO(philipj): legacycaller any (any... arguments); crbug.com/465009
 
     // obsolete members
     // https://html.spec.whatwg.org/#HTMLObjectElement-partial
     [Reflect] attribute DOMString align;
     [Reflect] attribute DOMString archive;
     [Reflect] attribute DOMString code;
     [Reflect] attribute boolean declare;
     // TODO(philipj): hspace and vspace should be unsigned long.
     [Reflect] attribute long hspace;
     [Reflect] attribute DOMString standby;
     [Reflect] attribute long vspace;
     [Reflect, URL] attribute DOMString codeBase;
     [Reflect] attribute DOMString codeType;
 
     [Reflect, TreatNullAs=EmptyString] attribute DOMString border;
 
     // TODO(philipj): These getters and setters are not in the spec.
     [Custom, NotEnumerable] getter boolean (unsigned long index);
     [Custom] setter boolean (unsigned long index, Node value);
     [Custom, NotEnumerable] getter Node (DOMString name);
     [Custom] setter Node (DOMString name, Node value);
 };