merge_pr_51525

Fix WPT scroll-start-overflow-toggled.tentative.html to properly await a requestAnimationFrame callback.

The pattern that the test was using, "await requestAnimationFrame", does
nothing because requestAnimationFrame doesn't return a Promise that could
be awaited.

To actually use 'await' to allow a requestAnimationFrame tick to pass, you need
to add your own Promise that gets resolved in requestAnimationFrame's callback,
as amended in this patch.

Differential Revision: https://phabricator.services.mozilla.com/D242551

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1955626
gecko-commit: 015e17880a341c5a017a978a16d3b8ff3de2ca9c
gecko-reviewers: emilio