Make it possible to custom-paint without an Element.

sky/engine/core/painting/Canvas.idl

Index: sky/engine/core/painting/Canvas.idl
diff --git a/sky/engine/core/painting/PaintingContext.idl b/sky/engine/core/painting/Canvas.idl
similarity index 63%
copy from sky/engine/core/painting/PaintingContext.idl
copy to sky/engine/core/painting/Canvas.idl
index 8eec203b5b9fbde6570dc4004f7244c640b03275..36fc06318df17e5002b04ca37245356e7bbf8a46 100644
--- a/sky/engine/core/painting/PaintingContext.idl
+++ b/sky/engine/core/painting/Canvas.idl
@@ -1,11 +1,11 @@
 // 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();
 };