Bug 1641952 [wpt PR 23860] - Fix calculation of before change style for CSS transitio...

Bug 1641952 [wpt PR 23860] - Fix calculation of before change style for CSS transitions, a=testonly

Automatic update from web-platform-tests
Fix calculation of before change style for CSS transitions

This patch performs a lazy calculation of the before change style once
it has been determined that a transition is being retargeted. In doing
so, the current position is correctly updated to reflect changes made
via the web-animation API.  The path also addresses a bug in the
calculation of current time, in the case of a paused or play-pending
animation.

Bug: 1082401, 888661, 547609

Change-Id: I4b76879d840b482da8ebf23c1aad41b881fafce8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2220263
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783675}

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wpt-commits: ebe69e97a1ea77b2eb30d7b40ffef2d67455a805
wpt-pr: 23860