[mediacapture-output] Should setSinkId be functional in SecureContext only?

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

== Should setSinkId be functional in SecureContext only? ==
Following on https://github.com/w3c/webrtc-pc/issues/1945, would it make sense to make *setSinkId* `[SecureContext]`? I.e.
```js
console.log("setSinkId" in document.createElement('video')); // false in http
```
We're looking for guidance on implementing it in Firefox.

Currently, in Chrome this method exists in http, but is pretty much [useless](http://jsfiddle.net/jib1/Lumb0w6e/) (`SecurityError`).

As to the read-only`sinkId` attribute, we could leave it (`""`) or make it SecureContext as well (`undefined`). With no way to affect the attribute, it serves little purpose.

Please view or discuss this issue at https://github.com/w3c/mediacapture-output/issues/78 using your GitHub account