Deflake a few scrollend and overscroll WPTs

This change replaces touch with mouse wheel as the method of scrolling
in

overscroll-deltas.html,
scrollend-event-fired-to-window.html, and
scrollend-event-fired-to-document.html

This allows these tests to be run on mac and also addresses an
issue affecting wpt.fyi[1] runs where webdriver is unhappy about the
touchpointer being out of bounds.

The increased timeouts addresses a flakiness I observed with rr when I
switched to using mouse which could possibly be related to
the 500ms timeout that mouse wheel scrollends are based on.

This patch adds some Math.round calls to scrollend-event-for-user-scroll.html because it appears the web driver version wpt.fyi uses does
not accept non int values for test_driver.Actions().pointerMove().

This patch also marks the wpt overscroll-deltas.html flaky.
We should revert this when crbug.com/1430921 is resolved.

[1]https://wpt.fyi/results/dom/events/scrolling?label=master&label=experimental&aligned&q=scrollend

Bug: 1429306,1430215,1430865,1430947
Change-Id: Icaa96202d2dd1adfc90c426c517fb9512affdc01
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4400771
Commit-Queue: David Awogbemila <awogbemila@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Reviewed-by: Mustaq Ahmed <mustaq@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1127417}