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.