Introduce toHTMLFormControlElement, and use it.

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 44 matching lines @@
     while (fieldset && !fieldset->hasTagName(fieldsetTag))
         fieldset = fieldset->parentElement();
     if (!fieldset)
         return 0;
 
     // Find first form element inside the fieldset that is not a legend element.
     // FIXME: Should we consider tabindex?
     Element* element = fieldset;
     while ((element = ElementTraversal::next(element, fieldset))) {
         if (element->isFormControlElement())
-            return static_cast<HTMLFormControlElement*>(element);
+            return toHTMLFormControlElement(element);
     }
 
     return 0;
 }
 
 void HTMLLegendElement::focus(bool, FocusDirection direction)
 {
     if (isFocusable())
         Element::focus(true, direction);
         
@@ skipping 14 matching lines @@
     // 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();
 }
     
 } // namespace