Bug 1827892 [wpt PR 39522] - [scroll-animations] Implement deferred timeline attachme...

Bug 1827892 [wpt PR 39522] - [scroll-animations] Implement deferred timeline attachment, a=testonly

Automatic update from web-platform-tests
[scroll-animations] Implement deferred timeline attachment

This CL implements the behavior behind the *-timeline-attachment
properties.

During CSSAnimations::CalculateTimelineUpdate, we now also calculate
the "attaching timelines", which means the deferred timelines in the
ancestor chain which correspond to the current element's timelines
with attachment=ancestor.

As usual, the pending changes to the "attaching timelines" are stored
on CSSAnimationUpdate, and only applied when (or if) CSSAnimations::
MaybeApplyPendingUpdate is called.

A couple of things are deferred to later CLs:

 - The behavior of ancestor-attached timelines with nothing to
   attach to. This is not clear yet.
 - Ensuring a sufficiently severe style recalc of children
   when the computed value of *-timeline-attachment property changes.

Bug: 1425939
Change-Id: I69ebc95c3936cb7276721d35b690e582188d9a9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4422650
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1130110}

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wpt-commits: 0bfffd48d12890b02caf178965a0f9b40567992b
wpt-pr: 39522