Re: Strawman Promises consensus position, based on Thursday's telechat

> To be fair, though, just claiming "people are using the prefixed variant"
> isn't totally representative - all they need is "navigator.getUserMedia =
> navigator.getUserMedia || navigator.webkitGetUserMedia ||
> navigator.mozGetUserMedia;" somewhere in their JS, and they can act as if
> it's unprefixed.  The Promises changeover will be a bit more impactful.

The prefixed versions can continue to support successCallback and
errorCallback to avoid this compatibility issue.

But yes, this is absolutely the right time to change this API to return a
promise, as all other specs are doing with async success/failure. If
getUserMedia goes 1.0 with callbacks, it'll be the first API to willfully
ignore promises, and without good reason.

If navigator.getUserMedia is a legacy navigator.mediaDevices.getUserMedia,
then remove the former from the spec now. The prefixed versions can
continue with the callbacks for compatibility.