Fortunately, Uber is getting into food delivery, so DoorDash will soon face a competitor without the inconvenience and awkwardness of tipping. Given Uber's popularity, as well as the trend of restaurants like Joe's Crab Shack ending tipping, I'd expect DoorDash to follow suit at some point.

Obligatory plug for http://reddit.com/r/EndTipping which discusses these issues, lists tip-free restaurants, etc.