Changing number of AudioDestinationNode outputs from 0 to 1

This came up in public-media-capture. Suppose you want to capture the
output of an AudioContext to a MediaStream or MediaRecorder. The natural
thing to do would be something like
  var d = context.createMediaStreamDestination();
  context.destination.connect(d);
or
  var r = new MediaRecorder(context.destination);
but that doesn't work because the AudioDestinationNode has no outputs.

So how about we make AudioDestinationNode output the AudioContext output
(the mix of its inputs)? Then the above code would work. I'm quite
confident it would be a Web-compatible change.

Rob
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