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 3f8e578d750ce4a709b3f0658a0b4b8a44e444ce..2fd765b0a2c97e5846b318a34db7a1da9c3475db 100644
--- a/net/quic/core/quic_session.cc
+++ b/net/quic/core/quic_session.cc
@@ -676,10 +676,9 @@ 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;
   }