Cleanup: Add conversion interface for WebWidget derived classes.

Source/web/WebPagePopupImpl.h

Index: Source/web/WebPagePopupImpl.h
diff --git a/Source/web/WebPagePopupImpl.h b/Source/web/WebPagePopupImpl.h
index a3c1cfb567f0a31f1d09fa7dcbe5633adad60261..74641ff31281b9faaef9dc1006daf3ad8a8a0320 100644
--- a/Source/web/WebPagePopupImpl.h
+++ b/Source/web/WebPagePopupImpl.h
@@ -73,6 +73,7 @@ private:
     virtual void close() OVERRIDE;
     virtual bool handleInputEvent(const WebInputEvent&) OVERRIDE;
     virtual void setFocus(bool) OVERRIDE;
+    virtual bool isPagePopup() const OVERRIDE { return true; }
 
     // PageWidgetEventHandler functions
     virtual bool handleKeyEvent(const WebKeyboardEvent&) OVERRIDE;
@@ -95,5 +96,18 @@ private:
     friend class PagePopupChromeClient;
 };
 
+inline WebPagePopupImpl* toWebPagePopupImpl(WebWidget* widget)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!widget || widget->isPagePopup());
+    return static_cast<WebPagePopupImpl*>(widget);
+}
+
+inline WebPagePopupImpl* toWebPagePopupImpl(WebCore::PagePopup* popup)
+{
+    // WebPagePopupImpl is the only implementation of WebCore::PagePopup, so
+    // no further checking required.
+    return static_cast<WebPagePopupImpl*>(popup);
+}
+
 } // namespace WebKit
 #endif // WebPagePopupImpl_h