[css-viewport] Remove assert for offsetTop after modifying zoom property (#45927)
There's no spec about what we should do with the scrollTop value
after applying zoom. Which makes Firefox fail the test:
css/css-viewport/zoom/scroll-top-test-with-zoom.html
Anyway checking that intermediate value is not relevant for this test,
that is checking that the scrollTop value doesn't change
when coming back to the initial zoom level.
So this patch removes the assert checking that value.
See https://bugzilla.mozilla.org/show_bug.cgi?id=1891910
for more details.