Engineers working on self-driving technology have given a lot of thought to difficult questions over the years, including how to keep pedestrians safe and how to avoid collisions with other vehicles. But last weekend’s protests in Los Angeles threw a spotlight on one of the thorny problems that remain for autonomous vehicles: what to do about arson, vandalism or other physical attacks. Five vehicles owned by Google’s self-driving spinoff Waymo were set ablaze last Sunday during protests agai...