Cleanup: Add conversion interface for WebWidget derived classes.

Source/web/WebHelperPluginImpl.h

 /*
  * Copyright (C) 2012 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 54 matching lines @@
 
 private:
     explicit WebHelperPluginImpl(WebWidgetClient*);
     bool initializePage(const String& pluginType, const WebDocument& hostDocument);
     void destroyPage();
 
     // WebWidget methods:
     virtual void layout() OVERRIDE;
     virtual void setFocus(bool) OVERRIDE;
     virtual void close() OVERRIDE;
+    virtual bool isHelperPlugin() const OVERRIDE { return true; }
 
     WebWidgetClient* m_widgetClient;
     WebViewImpl* m_webView;
     WebFrameImpl* m_mainFrame;
     OwnPtr<WebCore::Page> m_page;
     OwnPtr<HelperPluginChromeClient> m_chromeClient;
 
     friend class WebHelperPlugin;
     friend class HelperPluginChromeClient;
 };
 
+inline WebHelperPluginImpl* toWebHelperPluginImpl(WebWidget* widget)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!widget || widget->isHelperPlugin());
+    return static_cast<WebHelperPluginImpl*>(widget);
+}
+
 } // namespace WebKit
 
 #endif // WebHelperPluginImpl_h