Using unique_ptr of send side congestion controller (SSCC in
RtpTransportControllerSend. This prepares for a later CL where
the implementation of SSCC will be toggled by an experiment.
Bug: webrtc:8415
Change-Id: I52501d470efc96a62d0bd5c35ad361d43bceb9f9
Reviewed-on: https://webrtc-review.googlesource.com/58901
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22241}