[mediacapture-fromelement] Should tracks captured from a media element fire "ended" when ending? (#77)

Pehrsons has just created a new issue for https://github.com/w3c/mediacapture-fromelement:

== Should tracks captured from a media element fire "ended" when ending? ==
Spec reads:
> A captured MediaStreamTrack ends when [playback ends](http://www.w3.org/TR/html5/embedded-content-0.html#ended-playback) (and the ended event fires) or when the track that it captures is no longer selected or enabled for playback. A track is no longer selected or enabled if the source is changed by setting the src or srcObject attributes of the media element. The steps in [MediaStreamTrack.stop()](https://w3c.github.io/mediacapture-main/#dom-mediastreamtrack-stop) are performed on the MediaStreamTrack when it ends.

The steps for `MediaStreamTrack.stop()` do not fire "ended". IIRC this was motivated by there not being a need since the application knows it called stop anyway.

I would have expected us to fire "ended" here. It does seem like the original intention per https://github.com/w3c/mediacapture-fromelement/issues/23#issuecomment-277104443.

Please view or discuss this issue at https://github.com/w3c/mediacapture-fromelement/issues/77 using your GitHub account