It has become something of an online custom in the social media age to react to tragic news stories — like Wednesday’s attack in London — with well-meaning if sometimes rote messages like “thoughts and prayers”. But that does not appear to be Donald Trump Jr.’s style. “You have to be kidding me?!” Mr. Trump said Wednesday afternoon on Twitter, as details of the episode — which left at least five dead, including the assailant, and 40 injured — continued to unfold. The message continued, “Terror attacks are part of living in big city, says London Mayor Sadiq Khan”. Mr. Trump, the oldest son of President Trump, was calling attention to an article from September in The Independent, a British newspaper, that described Mr. Khan’s reaction to a bombing then in the Chelsea neighborhood of New York City.