Refactor wpt snap-at-user-scroll-end test to prevent flakiness

* Remove manual steps and UI (html) since test is automated
* Encapsulate test in promise_test to allow await for async operations.
As opposed to sequencing events using chained .then(). This improves
readability.
* Wait for wheel events before evaluating scroll completion to reduce
flakiness. Currently test waits for a scroll position to stop
changing rather than directly testing for scroll end. So the test
should ensure a wheel event was received before examining the scroll
position
* Wait for compositor resources to load before querying for them in
the javascript.

Bug: 40250245
Change-Id: I97dc80b174d976807c26a1bd058e5f845b6937d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6799109
Reviewed-by: David Awogbemila <awogbemila@chromium.org>
Commit-Queue: John An <johna@microsoft.com>
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1495578}