TBRs bypass owners checks, but are also semi-suspicious: a
contributor TBRing many changes in a row to submit code to
a directory they don't own would be seen, frowned upon, and
inquired into. However, a contributor could bypass this by
simply TBRing a single change to add themselves to an OWNERS
file, and then contributing as normal from there. This CL
removes that loophole.

This will not affect sheriffs who TBR reverts for two reasons:
first, it is rare that a chance touches both code and an
OWNERS file, and therefore it is rare that OWNERS changes
get reverted; second, quick reverts (the kind sheriffs do)
bypass PRESUBMIT entirely, and therefore also skip OWNERS
checks.

Bug: 688115
Change-Id: If2b5c9d058c62caf95389287e0bb706aef721baf
Reviewed-on: https://chromium-review.googlesource.com/982601
Commit-Queue: Aaron Gable <agable@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>