WASHINGTON — In a private call with federal prosecutors across the country, Attorney General William Barr’s message was clear: aggressively go after demonstrators who cause violence.Barr pushed his U.S. attorneys to bring federal charges whenever they could, keeping a grip on cases even if a defendant could be tried instead in state court, according to officials with knowledge of last week’s call who were not authorized to publicly discuss the matter and spoke on condition of anonymity. Federal convictions often result in …