Add QuicStrCat.

net/quic/core/quic_session.cc

Index: net/quic/core/quic_session.cc
diff --git a/net/quic/core/quic_session.cc b/net/quic/core/quic_session.cc
index cb75fe6835fcdb3bcc340507c82bada09a7b43d5..29d8de24bb543e9017ad1b460e9fda688bc1c7ec 100644
--- a/net/quic/core/quic_session.cc
+++ b/net/quic/core/quic_session.cc
@@ -12,6 +12,7 @@
 #include "net/quic/core/quic_connection.h"
 #include "net/quic/core/quic_flags.h"
 #include "net/quic/core/quic_flow_controller.h"
+#include "net/quic/platform/api/quic_str_cat.h"
 
 using base::IntToString;
 using base::StringPiece;
@@ -677,10 +678,9 @@ bool QuicSession::MaybeIncreaseLargestPeerStreamId(
              << " streams available, which would become "
              << new_num_available_streams << ", which exceeds the limit "
              << MaxAvailableStreams() << ".";
-    string details = IntToString(new_num_available_streams) + " above " +
-                     IntToString(MaxAvailableStreams());
     connection()->CloseConnection(
-        QUIC_TOO_MANY_AVAILABLE_STREAMS, details.c_str(),
+        QUIC_TOO_MANY_AVAILABLE_STREAMS,
+        QuicStrCat(new_num_available_streams, " above ", MaxAvailableStreams()),
         ConnectionCloseBehavior::SEND_CONNECTION_CLOSE_PACKET);
     return false;
   }