A protest outside a campaign rally for Donald J. Trump in Costa Mesa, Calif., turned violent late Thursday night, as the crowd smashed a police car, the authorities closed nearby streets and officers arrested about 20 people. The unrest erupted outside an amphitheater where Mr. Trump, the favorite in the race for the Republican presidential nomination, was addressing several thousand supporters, according to the Orange County Sheriff’s Department. There were no major injuries, and the Sheriff’s Department reported on Twitter that the protest was over and that the crowds had dispersed around 11 p.m., after officers on horseback pushed people...