Values:
ACL_MODE_UNKNOWN (0):
This value is required by protobuf best
practices
ACL_MODE_UNIVERSAL_ACCESS (1):
Universal Access: No document level access
control.
ACL_MODE_DOCUMENT_LEVEL_ACCESS_CONTROL_BYOID (2):
Document level access control with customer
own Identity Service.
ACL_MODE_DOCUMENT_LEVEL_ACCESS_CONTROL_GCI (3):
Document level access control using Google
Cloud Identity.
