Introduce toHTMLInputElement(Node*), and use it.

Source/core/html/HTMLInputElement.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, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2012 Samsung Electronics. 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
@@ skipping 409 matching lines @@
     bool m_canReceiveDroppedFiles : 1;
     bool m_hasTouchEventHandler : 1;
     OwnPtr<InputType> m_inputType;
     // The ImageLoader must be owned by this element because the loader code assumes
     // that it lives as long as its owning element lives. If we move the loader into
     // the ImageInput object we may delete the loader while this element lives on.
     OwnPtr<HTMLImageLoader> m_imageLoader;
     OwnPtr<ListAttributeTargetObserver> m_listAttributeTargetObserver;
 };
 
+inline HTMLInputElement* toHTMLInputElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(HTMLNames::inputTag));
+    return static_cast<HTMLInputElement*>(node);
+}
+
+inline const HTMLInputElement* toHTMLInputElement(const Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(HTMLNames::inputTag));
+    return static_cast<const HTMLInputElement*>(node);
+}
+
+// This will catch anyone doing an unnecessary cast.
+void toHTMLElement(const HTMLElement*);
+
+
 } //namespace
 #endif