Implement auto-roll script.

This script will (1) check if there is an active roll on rietveld (2) check if there is a trunk revision ready to be rolled and (3) call the chromium_roll script, creating a roll CL. The script will be called regularly through a cron job.

BUG=
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/212983003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20422 ce2b1a6d-e550-0410-aec6-3dcde31c8c00