Allow mocking SendSideCongestionController for Call tests.

webrtc/modules/congestion_controller/send_side_congestion_controller.cc

Index: webrtc/modules/congestion_controller/send_side_congestion_controller.cc
diff --git a/webrtc/modules/congestion_controller/send_side_congestion_controller.cc b/webrtc/modules/congestion_controller/send_side_congestion_controller.cc
index 66035b08588ac6d4faa3b127e80f9144835b392a..3d50bb9aae1475410c54d1fbda398b669fa3aaf7 100644
--- a/webrtc/modules/congestion_controller/send_side_congestion_controller.cc
+++ b/webrtc/modules/congestion_controller/send_side_congestion_controller.cc
@@ -46,23 +46,19 @@ static void ClampBitrates(int* bitrate_bps,
 
 SendSideCongestionController::SendSideCongestionController(
     const Clock* clock,
-    Observer* observer,
     RtcEventLog* event_log,
     PacketRouter* packet_router)
     : SendSideCongestionController(
           clock,
-          observer,
           event_log,
           std::unique_ptr<PacedSender>(
               new PacedSender(clock, packet_router, event_log))) {}
 
 SendSideCongestionController::SendSideCongestionController(
     const Clock* clock,
-    Observer* observer,
     RtcEventLog* event_log,
     std::unique_ptr<PacedSender> pacer)
     : clock_(clock),
-      observer_(observer),
       event_log_(event_log),
       pacer_(std::move(pacer)),
       bitrate_controller_(