Cleanup: Add conversion interface for WebWidget derived classes.

Source/web/WebPopupMenuImpl.h

Index: Source/web/WebPopupMenuImpl.h
diff --git a/Source/web/WebPopupMenuImpl.h b/Source/web/WebPopupMenuImpl.h
index f2decbb1b550a02fc0a1caf7e14b6504752a3931..29df9d17b58ac5cff37fed528aad3dad9b842c89 100644
--- a/Source/web/WebPopupMenuImpl.h
+++ b/Source/web/WebPopupMenuImpl.h
@@ -86,6 +86,7 @@ public:
     virtual bool caretOrSelectionRange(size_t* location, size_t* length) OVERRIDE;
     virtual void setTextDirection(WebTextDirection) OVERRIDE;
     virtual bool isAcceleratedCompositingActive() const OVERRIDE { return false; }
+    virtual bool isPopupMenu() const OVERRIDE { return true; }
 
     // WebPopupMenuImpl
     void initialize(WebCore::FramelessScrollView* widget, const WebRect& bounds);
@@ -134,6 +135,19 @@ public:
     WebCore::FramelessScrollView* m_widget;
 };
 
+inline WebPopupMenuImpl* toWebPopupMenuImpl(WebWidget* widget)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!widget || widget->isPopupMenu());
+    return static_cast<WebPopupMenuImpl*>(widget);
+}
+
+inline WebPopupMenuImpl* toWebPopupMenuImpl(WebCore::FramelessScrollViewClient* client)
+{
+    // WebPopupMenuImpl is the only implementation of FramelessScrollViewClient,
+    // so no need for further checking.
+    return static_cast<WebPopupMenuImpl*>(client);
+}
+
 } // namespace WebKit
 
 #endif