Populate slowstart numbers in stats.

net/quic/congestion_control/tcp_cubic_bytes_sender.cc

Index: net/quic/congestion_control/tcp_cubic_bytes_sender.cc
diff --git a/net/quic/congestion_control/tcp_cubic_bytes_sender.cc b/net/quic/congestion_control/tcp_cubic_bytes_sender.cc
index bd846730fe863e623309513ef19700762f1fc443..c9d71ee3bf614375f893efed9e0da22cef96b089 100644
--- a/net/quic/congestion_control/tcp_cubic_bytes_sender.cc
+++ b/net/quic/congestion_control/tcp_cubic_bytes_sender.cc
@@ -205,6 +205,10 @@ bool TcpCubicBytesSender::OnPacketSent(
     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;