As part of the work towards a single animation controller (https://bugs.webkit.org/show_bug.cgi?id=167899), we will need to migrate requestAnimationFrame to use it.
<rdar://problem/30461684>
An additional consideration is to make sure the new animation controller has a migration path to HTML 5.2 variable refresh rate standards, seen at https://github.com/w3c/html/issues/375 -- ala GSYNC, FreeSync, HDMI 2.1 VRR, VESA Adaptive-Sync (And possibly future versions of Apple ProMotion that supports dynamic VRR). e.g. Sychronizing refresh rate to the rate of animations of the dominant full screen canvas (e.g. 45fps makes the screen run at 45Hz), for <VIDEO>, for WebGL, and for Canvas2D.