Cleanup: Add conversion interface for WebWidget derived classes.

public/web/WebWidget.h

Index: public/web/WebWidget.h
diff --git a/public/web/WebWidget.h b/public/web/WebWidget.h
index f511ad39124ee82a81da644235ce804065258da8..73fcca58db780a0f49d03d55320a41a45b61db0a 100644
--- a/public/web/WebWidget.h
+++ b/public/web/WebWidget.h
@@ -220,6 +220,13 @@ public:
     // to render its contents.
     virtual bool isAcceleratedCompositingActive() const { return false; }
 
+    // Returns true if the WebWidget created is of type WebPagePopup.
+    virtual bool isPagePopup() const { return false; }
+    // Returns true if the WebWidget created is of type WebPopupMenu.
+    virtual bool isPopupMenu() const { return false; }
+    // Returns true if the WebWidget created is of type WebHelperPlugin.
+    virtual bool isHelperPlugin() const { return false; }
+
     // The WebLayerTreeView initialized on this WebWidgetClient will be going away and
     // is no longer safe to access.
     virtual void willCloseLayerTreeView() { }