Add QuicStrCat.

net/quic/core/quic_flow_controller_test.cc

Index: net/quic/core/quic_flow_controller_test.cc
diff --git a/net/quic/core/quic_flow_controller_test.cc b/net/quic/core/quic_flow_controller_test.cc
index 117540a0059e9d7cd1fd640b29e8f89cb67723b5..b3b166ff503b8709f6f0857272d801fff18c8737 100644
--- a/net/quic/core/quic_flow_controller_test.cc
+++ b/net/quic/core/quic_flow_controller_test.cc
@@ -8,6 +8,7 @@
 
 #include "base/format_macros.h"
 #include "base/strings/stringprintf.h"
+#include "net/quic/platform/api/quic_str_cat.h"
 #include "net/quic/test_tools/quic_connection_peer.h"
 #include "net/quic/test_tools/quic_flow_controller_peer.h"
 #include "net/quic/test_tools/quic_sent_packet_manager_peer.h"
@@ -81,8 +82,7 @@ TEST_F(QuicFlowControllerTest, SendingBytes) {
               CloseConnection(QUIC_FLOW_CONTROL_SENT_TOO_MUCH_DATA, _, _));
   EXPECT_QUIC_BUG(
       flow_controller_->AddBytesSent(send_window_ * 10),
-      base::StringPrintf("Trying to send an extra %" PRIu64 " bytes",
-                         send_window_ * 10));
+      QuicStrCat("Trying to send an extra ", send_window_ * 10, " bytes"));
   EXPECT_TRUE(flow_controller_->IsBlocked());
   EXPECT_EQ(0u, flow_controller_->SendWindowSize());
 }