Assign owners of smooth-scrolling and disable-threaded-scrolling flags.

Mark disable-threaded-scrolling non-expiring as it is a useful debugging
control when investigating scrolling issues.

Smooth scrolling was discussed on input-dev in https://goo.gl/qZ68P2 and
crbug.com/817814 tracks adding an accessibility setting for it, which we
should do before the flag expires.

Change-Id: I54fe4c4ff49b340af9b49d087e634f97d452a433
Reviewed-on: https://chromium-review.googlesource.com/c/1348868
Commit-Queue: Steve Kobes <skobes@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610856}