Oilpan: fix build++ after r192490.

Source/core/html/HTMLIFrameElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Simon Hausmann <hausmann@kde.org>
  * Copyright (C) 2004, 2006, 2008, 2009 Apple Inc. All rights reserved.
  *
  * 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.
@@ skipping 13 matching lines @@
 #ifndef HTMLIFrameElement_h
 #define HTMLIFrameElement_h
 
 #include "core/dom/DOMSettableTokenList.h"
 #include "core/html/HTMLFrameElementBase.h"
 
 namespace blink {
 
 class HTMLIFrameElement final : public HTMLFrameElementBase, public DOMSettableTokenListObserver {
     DEFINE_WRAPPERTYPEINFO();
+    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN_NESTED(HTMLIFrameElement, HTMLFrameElementBase);
 public:
     DECLARE_NODE_FACTORY(HTMLIFrameElement);
     DECLARE_VIRTUAL_TRACE();
     virtual ~HTMLIFrameElement();
     DOMSettableTokenList* sandbox() const;
 
 private:
     explicit HTMLIFrameElement(Document&);
 
     virtual void parseAttribute(const QualifiedName&, const AtomicString&) override;
@@ skipping 14 matching lines @@
     virtual void valueChanged() override;
 
     AtomicString m_name;
     bool m_didLoadNonEmptyDocument;
     RefPtrWillBeMember<DOMSettableTokenList> m_sandbox;
 };
 
 } // namespace blink
 
 #endif // HTMLIFrameElement_h