Use Gerrit (Polygerrit) for mini_chromium code review

As in Crashpad a4f3761de2de

Change-Id: Ib7f677768131e919a201614c1a0d09d58383e80f

Reviewed-on: https://chromium-review.googlesource.com/322150
Reviewed-by: Andrew Bonventre <andybons@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>