try_job can't be repeated, but buckets can. I think this does what
Ryan intended.

Also add a presubmit so this doesn't happen again.

No-try because CQ is broken, and even if it weren't nothing
would catch errors in cq.cfq directly.

No-try: True
Tbr: fbarchard@chromium.org
Bug: 869249
Change-Id: If84685f93275f5af691fc3fef77881156930b726
Reviewed-on: https://chromium-review.googlesource.com/1169820
Commit-Queue: Patrik Höglund <phoglund@chromium.org>
Reviewed-by: Patrik Höglund <phoglund@chromium.org>