Probing: Add support for exponential startup probing

webrtc/modules/congestion_controller/delay_based_bwe.h

Index: webrtc/modules/congestion_controller/delay_based_bwe.h
diff --git a/webrtc/modules/congestion_controller/delay_based_bwe.h b/webrtc/modules/congestion_controller/delay_based_bwe.h
index 1c4d02ff373d50764cebf1e3af23f841bf0f829b..b1453d656cd5932323dac1c18fe6efe2c66a9fb1 100644
--- a/webrtc/modules/congestion_controller/delay_based_bwe.h
+++ b/webrtc/modules/congestion_controller/delay_based_bwe.h
@@ -31,9 +31,11 @@
 
 namespace webrtc {
 
+class CongestionController;
+
 class DelayBasedBwe : public RemoteBitrateEstimator {
  public:
-  DelayBasedBwe(RemoteBitrateObserver* observer, Clock* clock);
+  DelayBasedBwe(CongestionController* controller, Clock* clock);
   virtual ~DelayBasedBwe() {}
 
   void IncomingPacketFeedbackVector(
@@ -57,15 +59,19 @@ class DelayBasedBwe : public RemoteBitrateEstimator {
 
  private:
   void IncomingPacketInfo(const PacketInfo& info);
+  // Updates the current remote rate estimate and returns true if a valid
+  // estimate exists.
+  bool UpdateEstimate(int64_t packet_arrival_time_ms,
+                      int64_t now_ms,
+                      int* target_bitrate_bps);
 
   rtc::ThreadChecker network_thread_;
   Clock* const clock_;
-  RemoteBitrateObserver* const observer_;
+  CongestionController* const controller_;
   std::unique_ptr<InterArrival> inter_arrival_;
   std::unique_ptr<OveruseEstimator> estimator_;
   OveruseDetector detector_;
   RateStatistics incoming_bitrate_;
-  int64_t first_packet_time_ms_;
   int64_t last_update_ms_;
   int64_t last_seen_packet_ms_;
   bool uma_recorded_;