Sheriff-o-matic should update periodically

We use the same update frequency as GOM does.

R=ojan@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/398143002

git-svn-id: svn://svn.chromium.org/blink/trunk@178292 bbb929c8-8fbe-4397-9dbb-9b2b20218538