WebRTC: Fix regression with sendonly streams
This MR fixes the regression in !1206 (merged) where sendonly streams don't start running.
There's one aspect that could still use attention - dropping unexpected RTP data for sendonly streams if we receive data anyway. That could happen with a weird peer, but more likely during renegotiation if the peer applies the SDP and starts sending data to us before we have applied the SDP.
That'll take more work to solve, but I'd like to get this merged to solve the regression.