Make it possible to custom-paint without an Element.

sky/engine/core/painting/Canvas.idl

 // 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.
-
-interface PaintingContext {
+interface Canvas {
+  // Height and width are used for culling optimizations and do not necessarily
+  // imply that the Canvas is backed by a buffer with any specific bounds.
   readonly attribute double height;
   readonly attribute double width;
 
   void drawCircle(double x, double y, double radius, Paint paint);
-  void commit();
 };