IsCurrent: Only return before-phase animations when they have positive playback rate

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

Bug: 1005848
Change-Id: I95981797ced813d6a7a0e57a1b719f3bb0bd736b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1814200
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699744}
6 files changed