Implement periodic bandwidth probing in application-limited region.

webrtc/modules/pacing/paced_sender.h

Index: webrtc/modules/pacing/paced_sender.h
diff --git a/webrtc/modules/pacing/paced_sender.h b/webrtc/modules/pacing/paced_sender.h
index ee50f2e4165145b3736a5c31c82dfbcdd68ea677..b9bb1c39a484da59142efc62b7c5a0254cf1a5b6 100644
--- a/webrtc/modules/pacing/paced_sender.h
+++ b/webrtc/modules/pacing/paced_sender.h
@@ -15,6 +15,7 @@
 #include <memory>
 #include <set>
 
+#include "webrtc/base/optional.h"
 #include "webrtc/base/thread_annotations.h"
 #include "webrtc/modules/include/module.h"
 #include "webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h"
@@ -120,12 +121,13 @@ class PacedSender : public Module, public RtpPacketSender {
   // packets in the queue, given the current size and bitrate, ignoring prio.
   virtual int64_t ExpectedQueueTimeMs() const;
 
-  // Application Limited Region refers to operating in a state where the
+  // Returns time in milliseconds when the current application-limited region
+  // started or empty result if the sender is currently not application-limited.
+  //
+  // Application Limited Region (ALR) refers to operating in a state where the
   // traffic on network is limited due to application not having enough
   // traffic to meet the current channel capacity.
-  //
-  // Returns true if network is currently application-limited.
-  bool InApplicationLimitedRegion() const;
+  virtual rtc::Optional<int64_t> GetApplicationLimitedRegionStartTime() const;
 
   // Returns the average time since being enqueued, in milliseconds, for all
   // packets currently in the pacer queue, or 0 if queue is empty.