Jean Liu has a vision. Not far in the future, the president of Didi Chuxing, the world’s largest ride-sharing service, imagines calling a car to her Beijing home. “I probably don’t even need to use this,” she says, holding up her smartphone. “Now it’s just some cool audio app that allows me to just say: “I need to go. Right now.” A vehicle will arrive a minute later. She need not specify what...