Populate FasterExtraStats for QUIC requests

net/quic/congestion_control/send_algorithm_interface.h

Index: net/quic/congestion_control/send_algorithm_interface.h
diff --git a/net/quic/congestion_control/send_algorithm_interface.h b/net/quic/congestion_control/send_algorithm_interface.h
index 98437ff9ceb8fdebc1a0316961f4fa12cd9ebf2c..080ff37c8613d6785e25478590fe7c4eb88b75c3 100644
--- a/net/quic/congestion_control/send_algorithm_interface.h
+++ b/net/quic/congestion_control/send_algorithm_interface.h
@@ -92,6 +92,11 @@ class NET_EXPORT_PRIVATE SendAlgorithmInterface {
   // not the *available* window.  Some send algorithms may not use a congestion
   // window and will return 0.
   virtual QuicByteCount GetCongestionWindow() const = 0;
+
+  // Returns the size of the slow start congestion window in bytes,
+  // aka ssthresh.  Some send algorithms do not define a slow start
+  // threshold and will return 0.
+  virtual QuicByteCount GetSlowStartThreshold() const = 0;
 };
 
 }  // namespace net