Add automatic tag script.

The script can be run in a cron job to automatically tag
lkgrs.

BUG=391261
LOG=n
TEST=tools/push-to-trunk/script_test.py
TEST=tools/push-to-trunk/auto_tag.py --dry_run
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/400693002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22477 ce2b1a6d-e550-0410-aec6-3dcde31c8c00