Clients of media_transport_interface need the ability to monitor BWE
estimates, and this change adds a TargetBitrate observer to the media
transport interface.
Bug: webrtc:9719
Change-Id: I90ebbf684c6f269e0c3cd58428010cfa511cc970
Reviewed-on: https://webrtc-review.googlesource.com/c/108106
Reviewed-by: Bjorn Mellem <mellem@webrtc.org>
Reviewed-by: Seth Hampson <shampson@webrtc.org>
Commit-Queue: Peter Slatala <psla@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25415}