Make WebNode::to<Type>() (and toConst) casts contain type asserts.

third_party/WebKit/Source/web/WebElement.cpp

 /*
  * Copyright (C) 2009 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 162 matching lines @@
         return WebImage();
 
     return WebImage(bitmap);
 }
 
 WebElement::WebElement(const PassRefPtrWillBeRawPtr<Element>& elem)
     : WebNode(elem)
 {
 }
 
+DEFINE_WEB_NODE_TYPE_CASTS(WebElement, isElementNode());
+
 WebElement& WebElement::operator=(const PassRefPtrWillBeRawPtr<Element>& elem)
 {
     m_private = elem;
     return *this;
 }
 
 WebElement::operator PassRefPtrWillBeRawPtr<Element>() const
 {
     return toElement(m_private.get());
 }
 
 } // namespace blink