Refactoring: Add convenience method toPluginContainerImpl.

Source/web/WebPluginContainerImpl.h

Index: Source/web/WebPluginContainerImpl.h
diff --git a/Source/web/WebPluginContainerImpl.h b/Source/web/WebPluginContainerImpl.h
index 5ecc64f243310669c3ffa7dfb7622e08ddda9ff7..d8e46d698e51d19926ea26d16c24ea3e364d9c96 100644
--- a/Source/web/WebPluginContainerImpl.h
+++ b/Source/web/WebPluginContainerImpl.h
@@ -53,6 +53,7 @@ class ResourceError;
 class ResourceResponse;
 class TouchEvent;
 class WheelEvent;
+class Widget;
 }
 
 namespace WebKit {
@@ -197,6 +198,24 @@ private:
     bool m_wantsWheelEvents;
 };
 
+inline WebPluginContainerImpl* toPluginContainerImpl(WebCore::Widget* widget)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!widget || widget->isPluginContainer());
+    // We need to ensure that the object is actually of type PluginContainer
+    // as there are many subclasses of Widget.
+    return static_cast<WebPluginContainerImpl*>(widget);
+}
+
+inline WebPluginContainerImpl* toPluginContainerImpl(WebPluginContainer* container)
+{
+    // Unlike Widget, we need not worry about object type for container.
+    // WebPluginContainerImpl is the only subclass of WebPluginContainer.
+    return static_cast<WebPluginContainerImpl*>(container);
+}
+
+// This will catch anyone doing an unnecessary cast.
+void toPluginContainerImpl(const WebPluginContainerImpl*);
+
 } // namespace WebKit
 
 #endif