The largest U.S. energy companies reported robust earnings on Friday, continuing a quarter in which the world’s big oil firms have posted some of their strongest gains since a pronounced price crash began in 2014. Exxon Mobil Corp. nearly doubled its second-quarter profit compared with a year ago, to $3.35 billion, and Chevron Corp. jacked up its bottom line to $1.45 billion. The gains came even as oil prices fell again in…