Populate slowstart numbers in stats.

net/quic/congestion_control/tcp_cubic_sender.cc

Index: net/quic/congestion_control/tcp_cubic_sender.cc
diff --git a/net/quic/congestion_control/tcp_cubic_sender.cc b/net/quic/congestion_control/tcp_cubic_sender.cc
index 0c2283e0012a8e0373822bf0d6dba86153406fee..e89666cb212233e63e8592a3eb4a6b5d0f65648f 100644
--- a/net/quic/congestion_control/tcp_cubic_sender.cc
+++ b/net/quic/congestion_control/tcp_cubic_sender.cc
@@ -205,6 +205,10 @@ bool TcpCubicSender::OnPacketSent(QuicTime /*sent_time*/,
                                   QuicPacketSequenceNumber sequence_number,
                                   QuicByteCount bytes,
                                   HasRetransmittableData is_retransmittable) {
+  if (InSlowStart()) {
+    ++(stats_->slowstart_packets_sent);
+  }
+
   // Only update bytes_in_flight_ for data packets.
   if (is_retransmittable != HAS_RETRANSMITTABLE_DATA) {
     return false;