builder_config: specify default gerrit_changes.

Some of the toolchain builders need to specify a default set of changes
to apply, for when they are launched by the scheduler.

BUG=chromium:1019868
TEST=generate

Change-Id: Idf5814317c63b53ec3303762f93454fbc2e1a311
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/1945919
Commit-Queue: LaMont Jones <lamontjones@chromium.org>
Tested-by: LaMont Jones <lamontjones@chromium.org>
Reviewed-by: Sean Abraham <seanabraham@chromium.org>
Reviewed-by: David Burger <dburger@chromium.org>
3 files changed