Introduce something like Gecko's refresh driver

[jdm]

From @bzbarsky:

It's basically a tick that we use to update our on-screen presentation.

When it ticks, we update the document timebase, resample animations/transitions, call requestAnimationFrame callbacks, recompute style and then layout, and paint.

We initially added it for the animation/transition bits and have been moving more and more things into there as time goes on. 

This makes sense, and we'll need something like this when we eventually get around to CSS transitions, video, etc.

[nox]

Didn't we implement this already?

[jdm]

We don't have a single unified mechanism, I don't think. We have individual messages for processing animations and rAF, and this is handled separately from styling and layout.

[mrobinson]

I think all that's required here is for advanceClock() to also trigger rAF callbacks. it already ticks animations, which should theoretically trigger a restyle and reflow.

[jdm]

The other difference we have right now is that we trigger layout at the end of every turn of the event loop (as well as manually in response to a bunch of things like mouse events and rAF callbacks). Part of introducing something like the refresh driver would be removing those, so that layout only happens in response to queries and the refresh driver.