Bug 1676761 [wpt PR 26142] - Initial implementation of progress based…

… ScrollTimelines, a=testonly

Automatic update from web-platform-tests
Initial implementation of progress based ScrollTimelines

This work is the first step in implementing upcoming spec change in:
w3c/csswg-drafts#4862

Converted AnimationTimeline::currentTime() to CSSNumberish
Added AnimationTimeline::duration

Bug: 1140602

Change-Id: I5c9b8b7130b91faae5f172493ba5f55d77b4bdd6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2476900
Commit-Queue: Jordan Taylor <[email protected]>
Reviewed-by: Robert Flack <[email protected]>
Cr-Commit-Position: refs/heads/master@{#826533}

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wpt-commits: d494b8ec23759c6bfb747bee1ff3a1b3d67f5177
wpt-pr: 26142