How to handle specs that extend MediaStreamTrack, in particular MediaStreamTrack slots?

[youennf]

mst content h defines a content hint extension to MediaStreamTrack.
But it does not define what should be done when cloning or transferring the track.
We should try to solve this.

[youennf]

One solution, mentioned by @jan-ivar is to define transfer and clone steps.
Then extension specs can add transfer/clone steps dedicated to their extension slots.

[jan-ivar]

Note this is about extension through partial interface MediaStreamTrack which seems largely editorial.

But a similar problem appears to exist with extension through inheritance, e.g. w3c/mediacapture-fromelement#96 but that's perhaps separate.