Calling git number requires that that depot tools is on the path so that
git-number can be found and requires the CHROME_HEADLESS environment
variable to be set.

Change-Id: Iabb8ec6732ca2bcc0f3209449e23a7d0a4f629e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3314033
Auto-Submit: Garrett Beaty <gbeaty@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>