Use ComputedStyle::HasCurrentCompositableAnimation for snapshots

During CSSAnimations::CalculateCompositorAnimationUpdate, we call
ComputedStyle::ShouldCompositeForCurrentAnimations as a proxy for
determining whether or not we may have any relevant animations
on the compositor that needs updating.

However, this function does not include background-color animations,
which can cause an incorrect early-out from CalculateCompositor-
AnimationUpdate, if background-color is the only running
compostiable animation.

This CL adds a new function on ComputedStyle which takes background-
color animations into account, and uses that in CalculateCompositor-
AnimationUpdate.

Bug: 1180159
Change-Id: Ic2a6723c93cec2f678695595998592ef55f9d903
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3138793


Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#918546}