bump grootfs
bump grootfs

[finishes #154801849]

Submodule src/code.cloudfoundry.org/grootfs c3be0dd..7960ac0:
  > Emit DownloadedLayersSizeInBytes metric

Signed-off-by: Will Martin <9c35c700d580163e024ea0f314140dbb6ef9e1ac@pivotal.io>

cloudfoundry-incubator/guardian-release,cloudfoundry-incubator/guardian-release,cloudfoundry-incubator/guardian-release