The Oak Brook-based chain known for food like Italian beef, Chicago dogs and chocolate cake shakes is just the latest restaurant to invest in delivery to meet growing demand, particularly among millennial consumers. Astute Portillo’s customers may have noticed the chain quietly ramping up the DoorDash delivery option in recent weeks, working out the kinks before making it official this week. The service is now available in most of the Chicago area, CEO Keith Kinsey said.