Implement periodic bandwidth probing in application-limited region.

webrtc/modules/pacing/alr_detector.h

Index: webrtc/modules/pacing/alr_detector.h
diff --git a/webrtc/modules/pacing/alr_detector.h b/webrtc/modules/pacing/alr_detector.h
index 31b1644d3020fc90a47b920378e91e1f5f9a4dc4..d7beedbe35238c29b14193d6734332b32dcdea1c 100644
--- a/webrtc/modules/pacing/alr_detector.h
+++ b/webrtc/modules/pacing/alr_detector.h
@@ -35,13 +35,16 @@ class AlrDetector {
   // Set current estimated bandwidth.
   void SetEstimatedBitrate(int bitrate_bps);
 
-  // Returns true if currently in application-limited region.
-  bool InApplicationLimitedRegion() const;
+  // Returns time in milliseconds when the current application-limited region
+  // started or empty result if the sender is currently not application-limited.
+  rtc::Optional<int64_t> GetApplicationLimitedRegionStartTime() const;
 
  private:
   RateStatistics rate_;
   int estimated_bitrate_bps_ = 0;
-  bool application_limited_ = false;
+
+  // Non-zero in ALR state.
+  int64_t alr_started_time_ms_ = 0;
 };
 
 }  // namespace webrtc