Track when QUIC connections are application limited. Protected by quic_enable_app_limited_check.

net/quic/core/congestion_control/send_algorithm_interface.h

Index: net/quic/core/congestion_control/send_algorithm_interface.h
diff --git a/net/quic/core/congestion_control/send_algorithm_interface.h b/net/quic/core/congestion_control/send_algorithm_interface.h
index 8869ac265f43b3c55120b2437bbfbe6be795007a..b9c1d4cbfd972982a061cc594ad84240b87bbec7 100644
--- a/net/quic/core/congestion_control/send_algorithm_interface.h
+++ b/net/quic/core/congestion_control/send_algorithm_interface.h
@@ -121,6 +121,20 @@ class NET_EXPORT_PRIVATE SendAlgorithmInterface {
   // Retrieves debugging information about the current state of the
   // send algorithm.
   virtual std::string GetDebugState() const = 0;
+
+  // Called when the connection has no outstanding data to send. Specifically,
+  // this means that none of the data streams are write-blocked, there are no
+  // packets in the connection queue, and there are no pending retransmissins,
+  // i.e. the sender cannot send anything for reasons other than being blocked
+  // by congestion controller. This includes cases when the connection is
+  // blocked by the flow controller.
+  //
+  // The fact that this method is called does not necessarily imply that the
+  // connection would not be blocked by the congestion control if it actually
+  // tried to send data. If the congestion control algorithm needs to exclude
+  // such cases, it should use the internal state it uses for congestion control
+  // for that.
+  virtual void OnApplicationLimited(QuicByteCount bytes_in_flight) = 0;
 };
 
 }  // namespace net