addTrack/removeTrack on gUM streams and PeerConnection remote streams

I can see why addTrack and removeTrack make sense for MediaStreams created
via one of the MediaStream constructors.

I don't see why it makes sense to be able to add arbitrary tracks to a
MediaStream created via getUserMedia or a PeerConnection remote stream.
That seems confusing and adds implementation complexity. I think it would
make more sense for a given MediaStream's track set to be managed either by
the UA or by script, but not both at the same time.

Rob
-- 
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