This change also adds a target_ref flag to presubmit_support.py.

Recipe-Nontrivial-Roll: build
Change-Id: I6de6bb87fc1482b88d9fbebe5e4ad1dbd8ce9748
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2702792
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>