demo.js assumes that unprefixed MediaStream is Firefox-only

https://github.com/addyosmani/getUserMedia.js/blob/a0f839a214f1143a66b44410804fbb1c1e712158/face-detection-demo/js/demo.js#L124

This code uses `(typeof MediaStream !== "undefined" && MediaStream !== null) && stream instanceof MediaStream` as the guard for a Firefox-only code path, where either `video.mozSrcObject` must be supported or one must support assigning a MediaStream instance to `video.src`. Neither on a path to standardization, only `srcObject` is.

This has broken the demo in Chrome 55, where I exposed the unprefixed MediaStream interface:
https://bugs.chromium.org/p/chromium/issues/detail?id=674608
https://bugs.chromium.org/p/chromium/issues/detail?id=649331

@alvestrand @henbos @guidou FYI