Add interface for AcknowledgedBitrateEstimator
Add simplified throughput estimator, implementing the same interface.
The choice of estimator implementation can be controlled by a field trial.
Bug: webrtc:10274
Change-Id: I6bef090a8a6a1783f3f5750a2ee56189f562a9c8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/158892
Commit-Queue: Björn Terelius <terelius@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29761}
