[css-viewport] Remove assert for offsetTop after modifying zoom prope…

…rty (#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.

css/css-viewport/zoom/scroll-top-test-with-zoom.html

@@ -23,9 +23,7 @@
   }, "Initial scrollTop with no zoom");
 
   document.body.style.zoom = 1.2;
-  test(function() {
-    assert_approx_equals(container.scrollTop, 77, 0.99, "scrollTop should remain consistent within 1 px after zooming in");
-  }, "scrollTop after increasing zoom level");
+  document.body.offsetTop;
 
   document.body.style.zoom = 1;
   test(function() {