The flaky test `input-value-does-not-force-layout.html` had an absolute
threshold of 5000ms before it would fail, which did not work that great
on CI. Since we are dealing with wildly different performance
characteristics on our CI runners (and ASAN enabled), make this test
perform relative timing comparisons instead.
Confirmed to still fail locally with the original fix removed.
We were mimicking Firefox' behavior that whenever a programmatic change
to an <input>'s or <textarea>'s selection happened, the new selection
focus is brought into view by scrolling. Currently we run a layout
update synchronously for that to make sure we have the fragment's
correct dimensions, which caused a significant performance regression in
Speedometer.
Since this is non-standard behavior, let's mimic Chromium instead which
does not scroll at all - only for direct user initiated input such as
typing.
Relevant issues:
* https://github.com/whatwg/html/issues/6217
* https://bugzilla.mozilla.org/show_bug.cgi?id=232405
* https://issues.chromium.org/issues/41081857
