When backing down, the delay based estimator can still use acked bitrate instead of the last set estimate.
Original code was reviewed in:
https://webrtc-review.googlesource.com/c/src/+/113880
BUG=webrtc:10144
Change-Id: Ia6e2d6d7d05f88f7e51d61b6e37c61a89adccf8a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/135950
Reviewed-by: Christoffer Rodbro <crodbro@webrtc.org>
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27912}