Use new Traversal<> API more in the code base

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 42 matching lines @@
 void HTMLFieldSetElement::trace(Visitor* visitor)
 {
 #if ENABLE(OILPAN)
     visitor->trace(m_associatedElements);
 #endif
     HTMLFormControlElement::trace(visitor);
 }
 
 void HTMLFieldSetElement::invalidateDisabledStateUnder(Element& base)
 {
-    for (Element* element = ElementTraversal::firstWithin(base); element; element = ElementTraversal::next(*element, &base)) {
-        if (element->isFormControlElement())
-            toHTMLFormControlElement(element)->ancestorDisabledStateWasChanged();
-    }
+    for (HTMLFormControlElement* element = Traversal<HTMLFormControlElement>::firstWithin(base); element; element = Traversal<HTMLFormControlElement>::next(*element, &base))
+        element->ancestorDisabledStateWasChanged();
 }
 
 void HTMLFieldSetElement::disabledAttributeChanged()
 {
     // This element must be updated before the style of nodes in its subtree gets recalculated.
     HTMLFormControlElement::disabledAttributeChanged();
     invalidateDisabledStateUnder(*this);
 }
 
 void HTMLFieldSetElement::childrenChanged(const ChildrenChange& change)
@@ skipping 52 matching lines @@
     }
 }
 
 const FormAssociatedElement::List& HTMLFieldSetElement::associatedElements() const
 {
     refreshElementsIfNeeded();
     return m_associatedElements;
 }
 
 } // namespace