Use DEFINE_NODE_TYPE_CASTS instead of using manual toHTMLFooElement()

Source/core/html/HTMLFieldSetElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2010 Apple Inc. All rights reserved.
  *
  * 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
  * version 2 of the License, or (at your option) any later version.
@@ skipping 45 matching lines @@
     virtual bool areAuthorShadowsAllowed() const OVERRIDE { return false; }
 
     static void invalidateDisabledStateUnder(Element*);
     void refreshElementsIfNeeded() const;
 
     mutable Vector<FormAssociatedElement*> m_associatedElements;
     // When dom tree is modified, we have to refresh the m_associatedElements array.
     mutable uint64_t m_documentVersion;
 };
 
-inline HTMLFieldSetElement* toHTMLFieldSetElement(Node* node)
-{
-    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(HTMLNames::fieldsetTag));
-    return static_cast<HTMLFieldSetElement*>(node);
-}
+DEFINE_NODE_TYPE_CASTS(HTMLFieldSetElement, hasTagName(HTMLNames::fieldsetTag));
 
 } // namespace
 
 #endif