Add GetDebugState() in SendAlgorithmInterface().

net/quic/core/quic_multipath_sent_packet_manager.cc

Index: net/quic/core/quic_multipath_sent_packet_manager.cc
diff --git a/net/quic/core/quic_multipath_sent_packet_manager.cc b/net/quic/core/quic_multipath_sent_packet_manager.cc
index a3f9320f4f8c9c0bb081b8e382ceef460ad2b9b2..ec3bb64f214ac1701e3ae1905f00e46ce2dffa95 100644
--- a/net/quic/core/quic_multipath_sent_packet_manager.cc
+++ b/net/quic/core/quic_multipath_sent_packet_manager.cc
@@ -316,6 +316,23 @@ QuicPacketCount QuicMultipathSentPacketManager::GetSlowStartThresholdInTcpMss()
   return path_manager->GetSlowStartThresholdInTcpMss();
 }
 
+string QuicMultipathSentPacketManager::GetDebugState() const {
+  string debug_state_by_path;
+  for (size_t i = 0; i < path_managers_info_.size(); ++i) {
+    if (path_managers_info_[i].manager == nullptr ||
+        path_managers_info_[i].state != ACTIVE) {
+      continue;
+    }
+    const string& debug_state = path_managers_info_[i].manager->GetDebugState();
+    if (debug_state.empty()) {
+      continue;
+    }
+    debug_state_by_path =
+        debug_state_by_path + "[" + base::IntToString(i) + "]:" + debug_state;
+  }
+  return debug_state_by_path;
+}
+
 void QuicMultipathSentPacketManager::CancelRetransmissionsForStream(
     QuicStreamId stream_id) {
   for (PathSentPacketManagerInfo path_manager_info : path_managers_info_) {