Pittsburgh Mayor Bill Peduto was at home with his girlfriend on New Year’s Eve when he glanced at his smartphone and saw a Facebook posting of a photograph of Police Chief Cameron McLay holding a sign reading “I resolve to challenge racism @ work. # end white silence.” “I thought, ‘What a great way to begin the new year,’ ” the mayor said, and he showed his girlfriend the photo. It had been taken by activists from What’s Up?! Pittsburgh, who approached the chief in a coffee shop during the city’s First Night festivities and asked him pose with their...