chromium_build_stats: disable vet in presubmit.
chromium_build_stats: disable vet in presubmit.

goapp vet doesn't work for now.
see https://code.google.com/p/googleappengine/issues/detail?id=11401

BUG=

Review URL: https://codereview.chromium.org/825353004

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra