Make it possible to custom-paint without an Element.

sky/engine/core/painting/PaintingTasks.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SKY_ENGINE_CORE_PAINTING_PAINTINGTASKS_H_
 #define SKY_ENGINE_CORE_PAINTING_PAINTINGTASKS_H_
 
 #include "sky/engine/wtf/PassOwnPtr.h"
 #include "sky/engine/wtf/PassRefPtr.h"
 
 namespace blink {
 class DisplayList;
 class Element;
+class Node;
 class PaintingCallback;
 
 class PaintingTasks {
 public:
     static void enqueueRequest(PassRefPtr<Element>, PassOwnPtr<PaintingCallback>);
-    static void enqueueCommit(PassRefPtr<Element>, PassRefPtr<DisplayList>);
+    static void enqueueCommit(PassRefPtr<Node>, PassRefPtr<DisplayList>);
 
     static bool serviceRequests();
     static void drainCommits();
 
 private:
     PaintingTasks() = delete;
     ~PaintingTasks() = delete;
 };
 
 } // namespace blink
 
 #endif  // SKY_ENGINE_CORE_PAINTING_PAINTINGTASKS_H_