Donald Trump Jr. and Trump campaign senior adviser Kimberly Guilfoyle came prepared with snappy rejoinders for liberal protesters who might taunt them on Sunday at the University of California at Los Angeles, where they promoted Trump Jr.’s new book, “Triggered: How the Left Thrives on Hate and Wants to Silence Us.” But when unruly heckling drowned out the couple’s answers, leading the pair to walk offstage, it wasn’t leftists who cut the event short.