Fix WPT version of scroll-timeilne-snapshotting test

[chromium-wpt-export-bot]

The original test has implicit timing assumptions about when
events would be dispatched relative to animation frame timing. This
test was also failing on webkit, which supports SDA.

The updated test, has fewer moving parts (pun intended) and works with
or without the snapshot post layout experiment.

Fixing as part of Interop 2026

Bug: 384523570
Change-Id: I84c223d7ca8e8aa1c238bd5b099f94a8712c33aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7718838
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Olga Gerchikov <gerchiko@microsoft.com>
Commit-Queue: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1609258}

[wpt-pr-bot]

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

[foolip]

This ought to fix the timeout, but we shouldn't modify this PR:

Suggested change

	await runAndWaitForFrameUpdate(() => {
	timeline = new ScrollTimeline({source: scroller});
	});
	await runAndWaitForFrameUpdate(t.step_func(() => {
	timeline = new ScrollTimeline({source: scroller});
	}));

[foolip]

With these changes Chrome and Firefox pass the test, but Firefox is a timeout instead of failing fast:
https://staging.wpt.fyi/results/scroll-animations/scroll-timelines/scroll-timeline-snapshotting.html?diff&filter=ADC&run_id=5183547490959360&run_id=6047047343931392

@kevers-google I've commented where I think the issue is. setup({ single_test: true }) could also be used to avoid the t.step_func wrapping, it would probably work if you combine it with <script type="module">.

Can you pick a fix you like in a follow-up Chromium CL or WPT PR?

[kevers-google]

A strategy adopted in later tests is to start with something like:

assert_implements(CSS.supports('animation-timeline:--foo');

or include a CSS rule like

 /* Ensure stable expectations if feature is not supported */
@supports not (animation-timeline:--foo) {
   #element { animation-play-state: paused; 
}

at the start of the test, to ensure a deterministic failure path when the feature is not supported.