[css-animationworklet] Option to run in-sync with rAF

[majido]

From @RByers on September 22, 2016 13:10

@birtles argues that you may sometimes want to have an animation that is guaranteed to run in-sync with rAF on the main thread. Perhaps there should be an option on the animator for this?

Personally I'm not sure when you'd choose to use AnimationWorklet like this instead of just writing animations using rAF, but perhaps there are code sharing benefits?

Copied from original issue: WICG/animation-worklet#1