One unusually warm evening in April, Richard Liu revved his scooter through Beijing’s traffic-snarled streets alongside other delivery workers, and then personally handed food orders to surprised customers. Later that night, over spicy hotpot and ice-cold beer, the JD.com Inc. founder welcomed a pair of riders from two rival delivery firms to his company. The publicity stunt, broadcast on viral online videos, reignited a fight for China’s $80 billion-plus food delivery market. In just a few mont...