Remove onFrame callback

sky/sdk/lib/rendering/README.md

 Sky Rendering
 =============
 
 The Sky render tree is a low-level layout and painting system based on a
 retained tree of objects that inherit from [`RenderObject`](object.dart). Most
 developers using Sky will not need to interact directly with the rendering tree.
 Instead, most developers should use [Sky widgets](../widgets/README.md), which
 are built using the render tree.
 
 Overview
@@ skipping 349 matching lines @@
 * Avoid using transforms where mere maths would be sufficient (e.g.
   draw your rectangle at x,y rather than translating by x,y and
   drawing it at 0,0).
 
 * Avoid using save/restore on canvases.
 
 
 Useful debugging tools
 ----------------------
 
 This is a quick way to dump the entire render tree to the console.

[Hixie, 2015/07/13 20:35:09]

add "every frame" at the end of this sentence.

 This can be quite useful in figuring out exactly what is going on when
 working with the render tree.
 
 ```dart
 import 'package:sky/rendering/sky_binding.dart';
+import 'package:sky/base/scheduler.dart' as scheduler;
 
-SkyBinding.instance.debugDumpRenderTree();
+scheduler.addPersistentFrameCallback((_) {
+  SkyBinding.instance.debugDumpRenderTree();
+});
 ```
 
 
 Dependencies
 ------------
 
  * [`package:sky/base`](../base)
  * [`package:sky/mojo`](../mojo)
  * [`package:sky/animation`](../mojo)