Snapshot post-layout for all but scroll-timelines

[chromium-wpt-export-bot]

'Run snapshot post-layout state steps' was introduced in the CSSOM View
spec[1] to formalize when state for scroll-state() queries, anchor
position effects based on scrolling, and scroll-timeline progress is
updated.

There is a pull request[2] to tell when to invoke snapshotting from the
HTML event loop. This CL aligns snapshotting with that CL for all
snapshotting except for scroll-timelines.

scroll-timeline-multi-pass.tentative.html is updated to align with the
one-snapshot-per-resizeObserver-loop-iteration in the HTML spec PR. The
difference between scroll-timelines and other snapshotting is that the
scroll-timelines are still snapshotted before the first style/layout
update in the resizeObserver loop.

The reason we keep the pre-layout snapshotting of scroll-timelines for
now is that timleines are only progressed before the first style/layout
update, which means the post-layout snapshotting will always cause
scroll-timelines to be a frame late (with the exception of the first
frame) unless we start allowing scroll-timelines to progress in the
resizeObserver loop.

[1] https://drafts.csswg.org/cssom-view/#run-snapshot-post-layout-state-steps
[2] whatwg/html#11613

Bug: 384523570
Change-Id: Iabb35fd06e5a2b9a95a09a25d27d7baad545653f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7673480
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1601971}

[wpt-pr-bot]

The review process for this patch is being conducted in the Chromium project.

[jonathan-j-lee]

WPT Command: python3 ./wpt run --channel=nightly --verify --verify-no-chaos-mode --verify-repeat-loop=0 --verify-repeat-restart=10 --github-checks-text-file=/home/test/artifacts/checkrun.md --affected base_head --log-mach-level=info --log-mach=- -y --no-pause --no-restart-on-unexpected --install-fonts --verify-log-full --no-headless --binary=/home/test/build/firefox/firefox firefox

Some affected tests had inconsistent (flaky) results:

Unstable results

Test	Subtest	Results	Messages
/scroll-animations/css/scroll-timeline-multi-pass.tentative.html		ERROR: 8/10, OK: 2/10

These may be pre-existing or new flakes. Please try to reproduce (see the above WPT command, though some flags may not be needed when running locally) and determine if your change introduced the flake. If you are unable to reproduce the problem, please tag @web-platform-tests/wpt-core-team in a comment for help.