toolchain: add PrepareForBuildResponse to chromiumos/common.

Begin migrating chromite.api.toolchain.PrepareForToolchainBuildResponse
to chromiumos.PrepareForBuildResponse

BUG=chromium:1019868
TEST=generate.sh

Change-Id: I5df8733284c8fb74658fb150a72ce1b10e7024d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/1972096
Tested-by: LaMont Jones <lamontjones@chromium.org>
Reviewed-by: David Burger <dburger@chromium.org>
Commit-Queue: LaMont Jones <lamontjones@chromium.org>
5 files changed