Add toHTMLFieldSetElement and toHTMLLegendElement, and use them.

Source/core/html/HTMLLegendElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2010 Apple Inc. All rights reserved.
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.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
@@ skipping 74 matching lines @@
 
 HTMLFormElement* HTMLLegendElement::virtualForm() const
 {
     // According to the specification, If the legend has a fieldset element as
     // its parent, then the form attribute must return the same value as the
     // form attribute on that fieldset element. Otherwise, it must return null.
     ContainerNode* fieldset = parentNode();
     if (!fieldset || !fieldset->hasTagName(fieldsetTag))
         return 0;
 
-    return static_cast<HTMLFieldSetElement*>(fieldset)->form();
+    return toHTMLFieldSetElement(fieldset)->form();
 }
 
 } // namespace