Republican front-runner Donald Trump, campaigning in California following fresh primary victories, called for party unity during an address at the state’s GOP convention while protesters denounced him and clashed with police outside. At the lunchtime gathering, Mr. Trump said he was best positioned to beat Democratic front-runner Hillary Clinton in a general election matchup. He said he would put into play New York, Florida, Michigan and Pennsylvania in November, states that have backed the Democratic nominee in most recent presidential elections. “The Republican Party, in a presidential sense, doesn’t win anymore,” Mr. Trump said. “You pick your standard cookie-cutters…I am...