Mark Attr.ownerElement as nullable

Source/core/dom/Attr.idl

 /*
  * Copyright (C) 2006, 2007, 2008, 2009 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 13 matching lines @@
 interface Attr : Node {
     readonly attribute DOMString? namespaceURI;
     readonly attribute DOMString? prefix;
     // FIXME: localName/name/value/nodeValue/textContent should not be nullable.
     readonly attribute DOMString? localName;
     readonly attribute DOMString? name;
     [ImplementedAs=valueForBindings, CustomElementCallbacks] attribute DOMString? value;
     [ImplementedAs=valueForBindings, MeasureAs=AttrNodeValue] attribute DOMString? nodeValue; // legacy alias of .value
     [ImplementedAs=valueForBindings, MeasureAs=AttrTextContent] attribute DOMString? textContent; // legacy alias of .value
 
-    // FIXME: ownerElement should be nullable.
-    [MeasureAs=AttributeOwnerElement] readonly attribute Element ownerElement;
+    [MeasureAs=AttributeOwnerElement] readonly attribute Element? ownerElement;
 
     [MeasureAs=AttributeSpecified] readonly attribute boolean specified; // useless; always returns true
 
     // FIXME: Attr should not have any child nodes. crbug.com/305105
     [MeasureAs=AttrChildAccess] readonly attribute NodeList childNodes;
     [MeasureAs=AttrChildAccess] readonly attribute Node firstChild;
     [MeasureAs=AttrChildAccess] readonly attribute Node lastChild;
 };