Adding getusermedia conformance tests.

So these are some tests we wrote a while back that I've updated and polished up so they follow the latest version of the spec. I'm not sure if we should rewrite them in the style of https://github.com/w3c/web-platform-tests/tree/master/mediacapture-streams, e.g. split them up a bit and perhaps enhance the existing tests instead.
<!doctype html> please :)
Otherwise, that one looks good. Maybe split the gUM tests and the webrtc one into two pull requests so that I can merge that one more quickly?