[mediacapture-viewport] Stop referencing DisplayMediaStreamConstraints dictionary (#21)

jan-ivar has just created a new issue for https://github.com/w3c/mediacapture-viewport:

== Stop referencing DisplayMediaStreamConstraints dictionary ==
Several recent non-constraint member additions to the *DisplayMediaStreamConstraints* dictionary are not relevant to *getViewportMedia*:
```idl
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.

Please view or discuss this issue at https://github.com/w3c/mediacapture-viewport/issues/21 using your GitHub account


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