Add const versions of some toHTMLFooElement().

Source/core/html/HTMLFormControlElement.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, 2007, 2008, 2009, 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 164 matching lines @@
     ASSERT_WITH_SECURITY_IMPLICATION(!node || (node->isElementNode() && toElement(node)->isFormControlElement()));
     return static_cast<HTMLFormControlElement*>(node);
 }
 
 inline HTMLFormControlElement* toHTMLFormControlElement(FormAssociatedElement* control)
 {
     ASSERT_WITH_SECURITY_IMPLICATION(!control || control->isFormControlElement());
     return static_cast<HTMLFormControlElement*>(control);
 }
 
+inline const HTMLFormControlElement* toHTMLFormControlElement(const FormAssociatedElement* control)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!control || control->isFormControlElement());
+    return static_cast<const HTMLFormControlElement*>(control);
+}
+
 } // namespace
 
 #endif