code: func (gce *Cloud) UpdateBackendService(bg *compute.BackendService) error {
  	op, err := gce.service.BackendServices.Update(gce.projectID, bg.Name, bg).Do()
  	if err != nil {
  		return err
  	}
  	return gce.waitForGlobalOp(op)
  }
docs: // UpdateBackendService applies the given BackendService as an update to an existing service.