Replace rAF wait with scrollend event wait

Previously there was a bug with scrollend where <input> elements did
not get scrollend events when scrolled. It has since been fixed so
touch-gesture-scroll-input-field.html can rely on scrollend events
which is a more reliable end-of-scroll signal than rAF waits.

Bug: 40250245
Change-Id: Ie2fd26624e1adad5036197e0cf17c0d360c668c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5631460


Commit-Queue: David Awogbemila <awogbemila@chromium.org>
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1316547}