MediaTrackSettings lacks support of channel layout

[henrikand]

Current specification supports channelCount

https://rawgit.com/w3c/mediacapture-main/master/getusermedia.html#def-constraint-channelCount

but this is not sufficient when the track contains more than two channels. In those cases, a channelLayout is also required [1].

[1] https://en.wikipedia.org/wiki/Surround_sound