Add critical flag to project_config.proto
Add critical flag to project_config.proto

For the passing of critical from builders to build, see https://chromium-review.googlesource.com/c/infra/infra/+/1861615

Bug:1013432
Change-Id: I67ab44b0833d46fd070afd8aa8909cbb40786659
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1861674
Commit-Queue: Quang Minh Tuan Nguyen <7f5d2f4f6ce87649784e35ad551490ab8d89e8eb@google.com>
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go