Pass --gerrit explicitly to be resiliant to possible rollbacks of the Gerrit
switch.

This'll also enforce using Gerrit on older release branches when using
the release tools for cherry-picking.

NOTRY=true
TBR=hablich@chromium.org

Bug: chromium:685318
Change-Id: If60784b4c804f38ca36649ac4b2e62209d7cf729
Reviewed-on: https://chromium-review.googlesource.com/565415Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46523}