Add an OpenChannel method to MediaTransportInterface and call it whenever PeerConnection opens a new data channel.

This informs the media transport that PeerConnection wants to use a data channel
and gives it a chance to set up before the data channel sends the first message.

Bug: webrtc:9719
Change-Id: I6ea905a74b29b8735e77ac68bc8606e7bca77f18
Reviewed-on: https://webrtc-review.googlesource.com/c/124020
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Peter Slatala <psla@webrtc.org>
Commit-Queue: Bjorn Mellem <mellem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26823}