[mediacapture-screen-share] Should getDisplayMedia be absent on platforms with no support? (#127)

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

== Should getDisplayMedia be absent on platforms with no support? ==
A [question came up](https://stackoverflow.com/questions/58842831/check-if-browser-platform-support-screen-capturing-via-getdisplaymedia) on stackoverflow: How to detect if screen sharing is supported on a particular browser and platform? Presumably this is to avoid showing users a non-functioning button.

It turns out mobile browsers like _Chrome for Android_ and _Firefox for Android_ define the API surface, but don't actually implement it, instead always rejecting with `NotAllowedError`.

This defeats feature-detection. Should those browsers instead have left `navigator.mediaDevices.getDisplayMedia` undefined?

Unlike with camera or microphone (which *is* detectable today, modulo discussions in https://github.com/w3c/mediacapture-main/issues/612 about *maybe* lying about this for privacy reasons), this would appear to leak no net fingerprinting surface about a user's system, since sites can already browser-sniff this from the UA string.

Please view or discuss this issue at https://github.com/w3c/mediacapture-screen-share/issues/127 using your GitHub account