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

third_party/WebKit/Source/web/WebPluginDocument.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 39 matching lines @@
     WebPluginContainerImpl* container = toWebPluginContainerImpl(doc->pluginWidget());
     return container ? container->plugin() : 0;
 }
 
 
 WebPluginDocument::WebPluginDocument(const PassRefPtrWillBeRawPtr<PluginDocument>& elem)
     : WebDocument(elem)
 {
 }
 
+DEFINE_WEB_NODE_TYPE_CASTS(WebPluginDocument, isDocumentNode() && constUnwrap<Document>()->isPluginDocument());
+
 WebPluginDocument& WebPluginDocument::operator=(const PassRefPtrWillBeRawPtr<PluginDocument>& elem)
 {
     m_private = elem;
     return *this;
 }
 
 WebPluginDocument::operator PassRefPtrWillBeRawPtr<PluginDocument>() const
 {
     return static_cast<PluginDocument*>(m_private.get());
 }
 
 } // namespace blink