Use waitForCSSScrollTimelineStyle() in pseudo-on-scroller.html.
It seems we use this function to make sure we get the correct computed values
for scroll animations in other tests. In this test, calling only one rAF
may cause the intermittents, so we use this function (i.e. call rAF twice)
to make sure the scroll animation is ready in the pseudo, and the
computed values have been updated properly, to avoid any intermittents.
Differential Revision: https://phabricator.services.mozilla.com/D218785
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1911967
gecko-commit: ff862a942918090068507576725c61f3c91c2d1f
gecko-reviewers: layout-reviewers, emilio