MediaStream integration

https://dvcs.w3.org/hg/audio/raw-file/tip/webaudio/webrtc-integration.html

Only being able to capture the output of an entire context seems
unnecessarily limiting. Instead of context.createMediaStreamDestination(),
you could add "readonly attribute MediaStream stream;" to AudioNode.

I think authors will expect createMediaStreamSource(stream).stream (or the
equivalent trivial graph using context.createMediaStreamDestination()) to
be semantically a no-op (except for the output having only one audio
track). I think we should make sure it is.

Rob
-- 
“You have heard that it was said, ‘Love your neighbor and hate your enemy.’
But I tell you, love your enemies and pray for those who persecute you,
that you may be children of your Father in heaven. ... If you love those
who love you, what reward will you get? Are not even the tax collectors
doing that? And if you greet only your own people, what are you doing more
than others?" [Matthew 5:43-47]