Some PRESUBMIT.py scripts set DEFAULT_WHITE_LIST and DEFAULT_BLACK_LIST.
This patch adds support to override those defaults during transition
period.

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

Bug: 1098562
Change-Id: I5dfff253e61485d354a83f3f509d8a6d4d87dec6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2270717Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>