NavigatorUserMedia no longer exists

[lukebjerring]

Found while authoring web-platform-tests/wpt#12284

getUserMedia in mediacapture-streams adds directly to Navigator.

So, this (mediacapture-screen-share) should be changed to

partial interface Navigator {
    Promise<MediaStream> getDisplayMedia(optional MediaStreamConstraints constraints);
};

[lukebjerring]

cc @foolip

[uysalere]

This can be a good opportunity to move it to MediaDevices as suggested on #55.

[foolip]

I'd like to stick to just fixing the IDL problem with this change. Note that the issue was closed and Edge has already shipped it on Navigator, so you'd need to reopen that discussion if you think this is important to change.