Add probing to recover faster from large bitrate drops.

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 429797af052d3234cb84478aa187241c39fec605..997d83adc726052446bf59df8dc55c819e614b0b 100644
--- a/webrtc/modules/congestion_controller/delay_based_bwe.h
+++ b/webrtc/modules/congestion_controller/delay_based_bwe.h
@@ -36,12 +36,16 @@ class DelayBasedBwe {
   static const int64_t kStreamTimeOutMs = 2000;
 
   struct Result {
-    Result() : updated(false), probe(false), target_bitrate_bps(0) {}
-    Result(bool probe, uint32_t target_bitrate_bps)
-        : updated(true), probe(probe), target_bitrate_bps(target_bitrate_bps) {}
+    Result();
+    Result(bool probe, uint32_t target_bitrate_bps);
+    Result(bool probe,
+           uint32_t target_bitrate_bps,
+           uint32_t suggested_probe_bps);
+    ~Result();
     bool updated;
     bool probe;
     uint32_t target_bitrate_bps;
+    rtc::Optional<uint32_t> suggested_probe_bps;
   };
 
   DelayBasedBwe(RtcEventLog* event_log, const Clock* clock);
@@ -60,9 +64,9 @@ class DelayBasedBwe {
  private:
   void IncomingPacketFeedback(const PacketFeedback& packet_feedback);
   Result OnLongFeedbackDelay(int64_t arrival_time_ms);
-
   Result MaybeUpdateEstimate(bool overusing,
-                             rtc::Optional<uint32_t> acked_bitrate_bps);
+                             rtc::Optional<uint32_t> acked_bitrate_bps,
+                             bool request_probe);
   // Updates the current remote rate estimate and returns true if a valid
   // estimate exists.
   bool UpdateEstimate(int64_t now_ms,
@@ -84,6 +88,8 @@ class DelayBasedBwe {
   double trendline_smoothing_coeff_;
   double trendline_threshold_gain_;
   int consecutive_delayed_feedbacks_;
+  uint32_t bitrate_before_last_large_drop_;
+  int64_t time_of_last_large_drop_ms_;
   uint32_t last_logged_bitrate_;
   BandwidthUsage last_logged_state_;
   bool in_sparse_update_experiment_;