This change corrects synchronization issues with animations, by
reworking the animation processing model to do a quick restyle and
incremental layout when ticking animations.

While this change adds overhead to animation ticks, the idea is that
this will be the fallback when synchronous behavior is required to
fulfill specification requirements. In the optimistic case, many
animations could be updated and applied off-the-main-thread and then
resynchronized when style information is queried by script.

Fixes #13865.