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.

[youennf]

Note though that CanvasCapture tracks have a canvas property that cannot be preserved. The requestFrame method is also probably not super useful when used in another context.

We should first decide what we want, and we will probably need to update the canvas capture spec in any case.