Add support for Bot-Review label to allow trusted bots to
bypass owners checks.

Bug: 1093627
Change-Id: Ie7398041e9e32fd87bb415b239b5ec0a4740e1ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2530516
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Yulan Lin <yulanlin@google.com>