BURLINGAME, Calif. — Donald J. Trump got a taste on Friday of what his next month of campaigning in California could be like. He was forced to exit his motorcade and walk through a field, climbing an embankment with Secret Service agents helping him, to avoid angry demonstrators on the street. “We went under a fence and through a fence, and oh, boy, it felt like I was crossing the border, actually,” Mr. Trump said when he finally made it to a ballroom to speak at California’s Republican Party convention. For the next 25 minutes, though, Mr. Trump spoke little...