Updated 'git-number' to produce an error message if it's run without the
CHROME_HEADLESS (bot) environment variable. This is intended to alert developers
who may be incorrectly using this tool to get positioning information instead of
the commit position.

BUG=409917
TEST=local
  - Ran with and without 'CHROME_HEADLESS'; observed the error message in the
    'without' case.

R=iannucci@chromium.org

Review URL: https://codereview.chromium.org/534703002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@291773 0039d316-1c4b-4281-b951-d872f2087c98