Uber has nailed down a record $3.5 billion investment from the Saudi Arabian Public Investment Fund — the largest-ever investment in a closely held technology firm. The investment is part of a deal that will allow Uber to expand its Middle East operations. Yasir Al Rumayyan, managing director of the fund, will take a seat on Uber’s board of directors. With a valuation of about $62.5 billion, Uber now is considered the most valuable venture capital-backed firm in the world.