This prepares for a later CL providing two implementations of SendSideCongestionController. Bug: webrtc:8415 Change-Id: I890dbe4b88bf609921558e03aac66b42629857c8 Reviewed-on: https://webrtc-review.googlesource.com/56700 Reviewed-by: Björn Terelius <terelius@webrtc.org> Commit-Queue: Sebastian Jansson <srte@webrtc.org> Cr-Commit-Position: refs/heads/master@{#22151}