Enable cron job with the given URN.

def EnableJob(self, job_id, token=None):
    """Enable cron job with the given URN."""
    job_urn = self.CRON_JOBS_PATH.Add(job_id)
    cron_job = aff4.FACTORY.Open(
        job_urn, mode="rw", aff4_type=CronJob, token=token)
    cron_job.Set(cron_job.Schema.DISABLED(0))
    cron_job.Close()