Reduce the horizontal sizes in WPT unreachable-snap-positions-003.html to avoid spurious failures in WPT harness with 600px-wide viewport.
This patch just halves the widths and the scrollLeft values in this test.
While we're at it, this patch also changes to use width rather than
max-width to size the scroller element, so that that element's sizing isn't
at-the-mercy of the viewport size. This patch also removes a 0.5x scale in the
meta-viewport tag, which was essentially a paper-over to reduce the widths of
the content here. That scale factor is no longer needed now that we've directly
made the content smaller.
The test still passes in all browsers (and now reliably passes in windows
at or smaller-than 600px wide).
The test also still fails in builds from before the associated fix landed (from
bug 1905247), so it still works as a regression test for that bug.
Differential Revision: https://phabricator.services.mozilla.com/D227994
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1929309
gecko-commit: 428c0fbb4a8c5f11782ad521d4fcec32b3870afd
gecko-reviewers: hiro