Nick Dunlap, a 20-year-old American who became the first player to win PGA Tour titles as an amateur and a professional in the same season, was named PGA Rookie of the Year on Thursday.Dunlap became the second-youngest player to capture the Arnold Palmer Award after Jordan Spieth, who was 20 years and two months when he took the honor in 2013 — nine months younger than Dunlap.