Add toHTMLMarqueeElement, and use it.

Source/core/html/HTMLMarqueeElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2007, 2010 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 49 matching lines @@
     // ActiveDOMObject
     virtual bool canSuspend() const;
     virtual void suspend(ReasonForSuspension);
     virtual void resume();
 
     virtual RenderObject* createRenderer(RenderStyle*) OVERRIDE FINAL;
 
     RenderMarquee* renderMarquee() const;
 };
 
+inline HTMLMarqueeElement* toHTMLMarqueeElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(HTMLNames::marqueeTag));
+    return static_cast<HTMLMarqueeElement*>(node);
+}
+
 } // namespace WebCore
 
 #endif // HTMLMarqueeElement_h