Transferring a MediaStreamTrack should preserve whether the object type

[youennf]

It is not clear from the spec whether transferring a MediaStreamTrack preserves the subtype. For instance, are we expecting transferring CanvasCaptureMediaStreamTrack to end up being a CanvasCaptureMediaStreamTrack? Probably, but we would probably need to state something like this in CanvasCaptureMediaStreamTrack spec. We could also state that it is expected that subtypes are preserved in media capture-extensions transfer section and refer to extension specs to define the additional steps required.