President-elect Donald J. Trump has offered Massad Boulos, a Lebanese American businessman and the father-in-law of his daughter Tiffany, a senior adviser role covering Arab and Middle Eastern affairs. Boulos played a major unofficial role in the Trump campaign, courting Arab American voters. He has already served as an informal liaison between Mr. Trump and Middle Eastern leaders. The position could prove influential as the incoming Trump administration faces various challenges in the region.