Without OS level locking it's possible to leave "lock" files on disk
which will prevent next run to acquire those locks. This can easily
happen if SIGKIL is issued.

R=apolito@google.com, ehmaldonado@chromium.org

Bug: 1049610
Change-Id: Id87aa1376b9ea5ff0c2d14f3603636493ed1dd5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2189333Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>