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 5b7981f1858bc685017c86477fd1f06c868ff171..041b07f26861a04f1512ba1ce30fd056ee1fc19f 100644
--- a/net/quic/core/quic_flow_controller_test.cc
+++ b/net/quic/core/quic_flow_controller_test.cc
@@ -10,6 +10,7 @@
 #include "base/strings/stringprintf.h"
 #include "net/quic/core/quic_flags.h"
 #include "net/quic/core/quic_utils.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"
@@ -84,8 +85,7 @@ 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());
 }