Refine zero-scroll-offset scroll anchoring heuristic to match blink.
See link in the comment. We were checking both offsets, but blink only
checks one.
In this case, the page has a table with overflow-x: auto and
overflow-y: hidden. They implement a custom virtual scrolling on the
vertical axis, and scroll-anchoring interacts poorly with it.
Adapt zero-scroll-offset.html to test for this.
Differential Revision: https://phabricator.services.mozilla.com/D217201
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1905426
gecko-commit: 6ccbb0036fca8c1f0091e50633504d91c9ea482f
gecko-reviewers: hiro