A rare scene in the American justice system unfolded Friday in a Chicago courthouse: A white officer stood before a mostly white jury and was convicted of killing a black teenager. It was the second such verdict nationally in two months. Jason Van Dyke’s guilty conviction for second-degree murder and multiple counts of aggravated battery for fatally shooting 17-year-old Laquan McDonald 16 times came two months after a Texas officer was convicted in the killing of a 15-year-old unarmed black boy.