"track" event shouldn't fire for a "recvonly" description.

[taylor-b]

This is something I'm pretty confident we decided previously. Consider the legacy addTrack use case, where PeerConnection A adds a track, but PeerConnection B doesn't, and they do an offer/answer exchange. The "track" event should fire for B, but not for A. Later, if B calls addTrack, and they do another offer/answer, the direction will change and the "track" event will fire for A.

But currently, the "dispatch a receiver" steps will unconditionally fire the "track" event for every new "m=" section.

[aboba]

@taylor-b It would be weird to fire a "track" event when in fact there is no incoming track. Can you submit a PR?