Remove DisplayMediaStreamConstraints use

[youennf]

DisplayMediaStreamConstraints has been removed from screen share spec.
We should have a ViewPortCaptureMediaStreamOptions instead of reusing/extending DisplayMediaStreamConstraints.

[dontcallmedom]

dup of #21 (for which #22 is pending)

[alvestrand]

The screen share renamed it, it did not remove it. We should switch to using the new name, not start inventing new stuff.

[dontcallmedom]

I don't think the options in DisplayMediaStreamOptions such as surfaceSwitching or selfBrowserSurface makes sense in the context of getViewportMedia, so using a dedicated dictionary as suggested in #22 seems preferable.