Add a merely pass-through QuicMultipathSentPacketManager. Protected behind blocked flag FLAGS_quic_…

net/quic/quic_connection.cc

Index: net/quic/quic_connection.cc
diff --git a/net/quic/quic_connection.cc b/net/quic/quic_connection.cc
index 0b4108df94e6588cfc1696f3e1479a81217d889f..ed58d1eb8ee6a0fdb3a2407f0becadfc6445e879 100644
--- a/net/quic/quic_connection.cc
+++ b/net/quic/quic_connection.cc
@@ -318,6 +318,10 @@ QuicConnection::QuicConnection(QuicConnectionId connection_id,
   framer_.set_received_entropy_calculator(&received_packet_manager_);
   last_stop_waiting_frame_.least_unacked = 0;
   stats_.connection_creation_time = clock_->ApproximateNow();
+  if (FLAGS_quic_enable_multipath) {
+    sent_packet_manager_.reset(new QuicMultipathSentPacketManager(
+        sent_packet_manager_.release(), this));
+  }
   // TODO(ianswett): Supply the NetworkChangeVisitor as a constructor argument
   // and make it required non-null, because it's always used.
   sent_packet_manager_->SetNetworkChangeVisitor(this);