Use Traversal<>::firstAncestor() API more in the code base

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 28 matching lines @@
     : HTMLElement(legendTag, document)
 {
     ScriptWrappable::init(this);
 }
 
 DEFINE_NODE_FACTORY(HTMLLegendElement)
 
 HTMLFormControlElement* HTMLLegendElement::associatedControl()
 {
     // Check if there's a fieldset belonging to this legend.
-    Element* fieldset = parentElement();
-    while (fieldset && !isHTMLFieldSetElement(*fieldset))
-        fieldset = fieldset->parentElement();
+    HTMLFieldSetElement* fieldset = Traversal<HTMLFieldSetElement>::firstAncestor(*this);
     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 17 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 (!isHTMLFieldSetElement(fieldset))
         return 0;
 
     return toHTMLFieldSetElement(fieldset)->formOwner();
 }
 
 } // namespace