Add GetDebugState() in SendAlgorithmInterface().

net/quic/core/congestion_control/tcp_cubic_sender_base.cc

Index: net/quic/core/congestion_control/tcp_cubic_sender_base.cc
diff --git a/net/quic/core/congestion_control/tcp_cubic_sender_base.cc b/net/quic/core/congestion_control/tcp_cubic_sender_base.cc
index caa02ebf92130797d2b0d29fcb8980c1a9d79395..9bd013fb9904a4654f32adaa5e73001b89506f37 100644
--- a/net/quic/core/congestion_control/tcp_cubic_sender_base.cc
+++ b/net/quic/core/congestion_control/tcp_cubic_sender_base.cc
@@ -5,6 +5,7 @@
 #include "net/quic/core/congestion_control/tcp_cubic_sender_base.h"
 
 #include <algorithm>
+#include <string>
 
 #include "base/metrics/histogram_macros.h"
 #include "net/quic/core/congestion_control/prr_sender.h"
@@ -283,4 +284,8 @@ void TcpCubicSenderBase::OnConnectionMigration() {
   last_cutback_exited_slowstart_ = false;
 }
 
+std::string TcpCubicSenderBase::GetDebugState() const {
+  return "";
+}
+
 }  // namespace net