Cleanup: Add conversion interface for WebWidget derived classes.

Source/web/WebHelperPluginImpl.h

Index: Source/web/WebHelperPluginImpl.h
diff --git a/Source/web/WebHelperPluginImpl.h b/Source/web/WebHelperPluginImpl.h
index d7ed9d3ebabd36b7437235991530e3ad8c70ed5a..93f38f994827037fe9618c4b343ae095470bf82f 100644
--- a/Source/web/WebHelperPluginImpl.h
+++ b/Source/web/WebHelperPluginImpl.h
@@ -72,6 +72,7 @@ private:
     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;
@@ -83,6 +84,12 @@ private:
     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