Re: addTrack/removeTrack on gUM streams and PeerConnection remote streams

To be concrete, here's what I think we should do:

-- Introduce a new subtype of MediaStream, let's call it BundleMediaStream
but I don't care what it's called. This stream represents a bundle of
tracks from other MediaStreams, where the application controls the track
set.
-- Move the current MediaStream constructors to BundleMediaStream.
-- Move addTrack/removeTrack to BundleMediaStream.
-- Specify that for MediaStreams other than BundleMediaStream, the UA
always controls the track set.

This means for any MediaStream, either the UA controls the track set, or
the application does, but not both. I think this is a helpful
simplification for implementations and at the spec level.

How does that sound?

A reasonable alternative that requires less new syntax would be to make
addTrack/removeTrack throw an exception when called on a MediaStream that
wasn't constructed via a MediaStream constructor. However, I think
introducing a new type is cleaner.

Rob
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