Web-animation spec [1] requires active animations to effectively act as if there is an equivalent will-change for its affected properties.

We currently don't do this but rather have specific logic for a composited animated properties that have *some* of the same effect (e.g., active transform animation will cause a layer to be created similar to will-change:transform.).

I suggest we match the spec here and remove the separate path that exists for animations.
This my also fix  issue 764783  and  issue 873673 .


[1] https://drafts.csswg.org/web-animations/#side-effects-section

"For every property targeted by at least one animation effect that is current or in effect, the user agent must act as if the will-change property ([css-will-change-1]) on the target element includes the property."