As lead singer of New York electro-glam revivalists Semi Precious Weapons, Justin Tranter toured with Lady Gaga and got a close-up view of 21st-century pop's inner workings. Fall Out Boy's crashing “Centuries” and Imagine Dragons' scream-along “Believer” show how his compositions can straddle the gap between rock attitude and pop grandeur, while Justin Bieber's loping “Sorry” and DNCE's strutting “Cake by the Ocean” prove his hooks' dance-floor potency.