This relands the safe portion of outage-causing https://chromium-review.googlesource.com/c/509451/

Landing this simple CL will allow pre-emptively adding escape hatch
to critical places (such as update_scripts) to make relanding validation safer.

Bug: 570091
Change-Id: I4d963bd9b140b7b6bca35d4d5e41768bc1a3d7e0
Reviewed-on: https://chromium-review.googlesource.com/512744
Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>