Introduce toHTMLSelectElement and remove isHTMLSelectElement

Source/web/WebSelectElement.cpp

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 56 matching lines @@
 }
 
 WebSelectElement& WebSelectElement::operator=(const PassRefPtr<HTMLSelectElement>& elem)
 {
     m_private = elem;
     return *this;
 }
 
 WebSelectElement::operator PassRefPtr<HTMLSelectElement>() const
 {
-    return static_cast<HTMLSelectElement*>(m_private.get());
+    return toHTMLSelectElement(m_private.get());
 }
 
 } // namespace WebKit