Add support for Owners-Override label to allow trusted bots, sheriffs,
and global owners to bypass owners checks.

Bug: 1093627
Change-Id: Ie217369a67e4060807c0a12f84430e30a7c57daa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2530488Reviewed-by: Yulan Lin <yulanlin@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>