Stop referencing DisplayMediaStreamConstraints dictionary

[jan-ivar]

Several recent non-constraint member additions to the DisplayMediaStreamConstraints dictionary are not relevant to getViewportMedia:

dictionary DisplayMediaStreamConstraints {
  (boolean or MediaTrackConstraints) video = true;
  (boolean or MediaTrackConstraints) audio = false;
  SelfCapturePreferenceEnum selfBrowserSurface; // 1
  SystemAudioPreferenceEnum systemAudio; // 2
  SurfaceSwitchingPreferenceEnum surfaceSwitching; // 3
};

This spec needs to maintain its own copy of this dictionary without these additions.

[eladalon1983]

I've sent a PR to this effect (#32).

I've gone ahead and included controller there. We might need to spruce up some of the language in the gDM spec, so that the controller would work equally well for both gDM and gVM. I think we can tackle that later, though.