Add toHTMLFieldSetElement and toHTMLLegendElement, and use them.

Source/core/html/HTMLFieldSetElement.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 76 matching lines @@
 
 RenderObject* HTMLFieldSetElement::createRenderer(RenderStyle*)
 {
     return new RenderFieldset(this);
 }
 
 HTMLLegendElement* HTMLFieldSetElement::legend() const
 {
     for (Element* child = ElementTraversal::firstWithin(this); child; child = ElementTraversal::nextSkippingChildren(child, this)) {
         if (child->hasTagName(legendTag))
-            return static_cast<HTMLLegendElement*>(child);
+            return toHTMLLegendElement(child);
     }
     return 0;
 }
 
 PassRefPtr<HTMLCollection> HTMLFieldSetElement::elements()
 {
     return ensureCachedHTMLCollection(FormControls);
 }
 
 void HTMLFieldSetElement::refreshElementsIfNeeded() const
@@ skipping 29 matching lines @@
 {
     refreshElementsIfNeeded();
     unsigned len = 0;
     for (unsigned i = 0; i < m_associatedElements.size(); ++i)
         if (m_associatedElements[i]->isEnumeratable())
             ++len;
     return len;
 }
 
 } // namespace