These MediaStreams are used as a way to down-mix the input AudioChunks, and
also as a way to get proper stream processing ordering. The MediaStream for
the source AudioNode is an input to these streams, and these streams in turn
are inputs to the MediaStream that the AudioNode that owns the AudioParam owns.
This way, the Media Streams Graph processing code will order the streams so
that by the time that the MediaStream for a given node is processed, all of the
MediaStreams belonging to the AudioNode(s) feeding into the AudioParam have
been processed.
This has a tricky side-effect that those streams also being considered when
determining the input block for the AudioNodeStream belonging to the
AudioParam's owner AudioNode. In order to fix that, we simply special case
those streams and make AudioNodeStream::ObtainInputBlock ignore them.
