The update mask applies to the resource. Only display_name,
description and parent_policy_tag can be updated and thus can
be listed in the mask. If update_mask is not provided, all
allowed fields (i.e. display_name, description and parent)
will be updated. For more information including the
FieldMask definition, see https://developers.google.com
/protocol-buffers/docs/reference/google.protobuf#fieldmask If
not set, defaults to all of the fields that are allowed to
update.