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.