Add QuicStrCat.

net/quic/core/quic_flow_controller.cc

Index: net/quic/core/quic_flow_controller.cc
diff --git a/net/quic/core/quic_flow_controller.cc b/net/quic/core/quic_flow_controller.cc
index 12c0368d4c172c63b8d5107e9a75157dac0a2634..202a4f3ac18597cc3781942e13a99095e7754513 100644
--- a/net/quic/core/quic_flow_controller.cc
+++ b/net/quic/core/quic_flow_controller.cc
@@ -77,11 +77,8 @@ void QuicFlowController::AddBytesSent(QuicByteCount bytes_sent) {
     // This is an error on our side, close the connection as soon as possible.
     connection_->CloseConnection(
         QUIC_FLOW_CONTROL_SENT_TOO_MUCH_DATA,
-        base::StringPrintf(
-            "%llu bytes over send window offset",
-            static_cast<unsigned long long>(send_window_offset_ -
-                                            (bytes_sent_ + bytes_sent)))
-            .c_str(),
+        QuicStrCat(send_window_offset_ - (bytes_sent_ + bytes_sent),
+                   "bytes over send window offset"),
         ConnectionCloseBehavior::SEND_CONNECTION_CLOSE_PACKET);
     return;
   }