Add GetDebugState() in SendAlgorithmInterface().

net/quic/core/quic_multipath_sent_packet_manager.h

Index: net/quic/core/quic_multipath_sent_packet_manager.h
diff --git a/net/quic/core/quic_multipath_sent_packet_manager.h b/net/quic/core/quic_multipath_sent_packet_manager.h
index 46fe740c9128c19c9875ed7cb95207433407a1cb..e1fc6b7273e951abb6d766ee3db0830dca10c23a 100644
--- a/net/quic/core/quic_multipath_sent_packet_manager.h
+++ b/net/quic/core/quic_multipath_sent_packet_manager.h
@@ -132,6 +132,10 @@ class NET_EXPORT_PRIVATE QuicMultipathSentPacketManager
   // TCP segments on the default path.
   QuicPacketCount GetSlowStartThresholdInTcpMss() const override;
 
+  // Returns debugging information about the state of the congestion
+  // controller for all paths.
+  std::string GetDebugState() const override;
+
   // No longer retransmit data for |stream_id| on all paths and any pending
   // retransmissions in pending_retransmissions_.
   void CancelRetransmissionsForStream(QuicStreamId stream_id) override;