Push back on the video encoder to avoid building queues in the pacer.

Implemented behind the field trial "WebRTC-PacerPushbackExperiment/Enabled/"

BUG=webrtc:8171, webrtc:8287

Review-Url: https://codereview.webrtc.org/3004783002
Cr-Commit-Position: refs/heads/master@{#19969}

modules/congestion_controller/include/send_side_congestion_controller.h

@@ -154,6 +154,9 @@ class SendSideCongestionController : public CallStatsObserver,
 
   rtc::RaceChecker worker_race_;
 
+  bool pacer_pushback_experiment_ = false;
+  float encoding_rate_ = 1.0;
+
   RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(SendSideCongestionController);
 };