Populate FasterExtraStats for QUIC requests

net/quic/quic_sent_packet_manager.h

Index: net/quic/quic_sent_packet_manager.h
diff --git a/net/quic/quic_sent_packet_manager.h b/net/quic/quic_sent_packet_manager.h
index 89a3b6d26a93e6ce344ab6d1fb1254cdaf3fef11..9c2f3c20d2c555d23029c18a8420555c5017fc87 100644
--- a/net/quic/quic_sent_packet_manager.h
+++ b/net/quic/quic_sent_packet_manager.h
@@ -173,6 +173,11 @@ class NET_EXPORT_PRIVATE QuicSentPacketManager {
   // window and will return 0.
   QuicByteCount GetCongestionWindow() const;
 
+  // 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.
+  QuicByteCount GetSlowStartThreshold() const;
+
   // Enables pacing if it has not already been enabled, and if
   // FLAGS_enable_quic_pacing is set.
   void MaybeEnablePacing();