Fix IsCurrent to return after-phase animations with negative playback rate

See https://drafts.csswg.org/web-animations-1/#current

Bug: 1005852
Change-Id: I76390e0bc99078b5c1644970ef1985492414e6e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1814638
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699779}
2 files changed