Introduce something like Gecko's refresh driver #3406
Description
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.