logging.warn is deprecated and should be replaced with logging.warning.

Bug:1097402
Change-Id: I55ce007bec47608eeafcf3cd9ac2b90f60073115
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2575621Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>