PALO ALTO, Calif. — Donald J. Trump would not be Silicon Valley’s first choice as president. Or its second. Or maybe even its third. The presumptive Republican presidential nominee wants to restrict immigration while bringing back manufacturing. He compulsively uses tech products like Twitter but is not in awe of the people who built them. He made his fortune the old-fashioned way, by going into the family business, in the old-fashioned industry of real estate. He’s not the valley’s kind of entrepreneur. Worst of all, Mr. Trump is revealing Silicon Valley’s vulnerability. In recent years, technology companies have extended their...