Use new is*Element() helper functions further more in HTML code

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 32 matching lines @@
 
 PassRefPtr<HTMLLegendElement> HTMLLegendElement::create(Document& document)
 {
     return adoptRef(new HTMLLegendElement(document));
 }
 
 HTMLFormControlElement* HTMLLegendElement::associatedControl()
 {
     // Check if there's a fieldset belonging to this legend.
     Element* fieldset = parentElement();
-    while (fieldset && !fieldset->hasTagName(fieldsetTag))
+    while (fieldset && !isHTMLFieldSetElement(*fieldset))
         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?
     return Traversal<HTMLFormControlElement>::next(*fieldset, fieldset);
 }
 
 void HTMLLegendElement::focus(bool, FocusType type)
@@ skipping 11 matching lines @@
     if (HTMLFormControlElement* control = associatedControl())
         control->accessKeyAction(sendMouseEvents);
 }
 
 HTMLFormElement* HTMLLegendElement::form() 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))
+    if (!isHTMLFieldSetElement(fieldset))
         return 0;
 
     return toHTMLFieldSetElement(fieldset)->formOwner();
 }
 
 } // namespace