Introduce toHTMLFormControlElement, and use it.

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 153 matching lines @@
 
     // Cache of validity()->valid().
     // But "candidate for constraint validation" doesn't affect m_isValid.
     bool m_isValid : 1;
 
     bool m_wasChangedSinceLastFormControlChangeEvent : 1;
 
     bool m_hasAutofocused : 1;
 };
 
+inline HTMLFormControlElement* toHTMLFormControlElement(Node* node)
+{
+    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);
+}
+
 } // namespace
 
 #endif