Refactoring: Add convenience method toPluginContainerImpl.

Source/web/WebNode.cpp

Index: Source/web/WebNode.cpp
diff --git a/Source/web/WebNode.cpp b/Source/web/WebNode.cpp
index 1b41d253fc993848d5140ae0de98230d98a74fb2..e2a984b5f79e19801c092f8eaf605974419f90fb 100644
--- a/Source/web/WebNode.cpp
+++ b/Source/web/WebNode.cpp
@@ -243,7 +243,7 @@ WebPluginContainer* WebNode::pluginContainer() const
         if (object && object->isWidget()) {
             Widget* widget = WebCore::toRenderWidget(object)->widget();
             if (widget && widget->isPluginContainer())
-                return static_cast<WebPluginContainerImpl*>(widget);
+                return toPluginContainerImpl(widget);
         }
     }
     return 0;