Add the Auto-Submit label definition as well

The previous CL just granted access to the label, but didn't actually
define the label.

Change-Id: I3cffbe71a2492b2efeab15db37b432c8ca13cd46
Reviewed-on: https://dart-review.googlesource.com/c/All-Projects/+/150042
Reviewed-by: William Hesse <whesse@google.com>