Clarify in which animation frame task events are fired

[foolip]

https://w3c.github.io/screen-orientation/#handling-screen-orientation-changes

Although animation frame tasks aren't actually defined properly yet, they will have to be a per-browsing context concept, similar to window.requestAnimationFrame().

For screen orientation, it hopefully works to change the structure to "for each animation frame task enqueue an animation frame task to ...". In the absence of a spec, the discussion in whatwg/html#707 (comment) might be helpful.

[xfq]

Given that whatwg/html#2627 has been closed, mark this issue as "future" for now.

[annevk]

Let's fold this into #136.