Add QuicStrCat.

net/quic/core/quic_connection_test.cc

Index: net/quic/core/quic_connection_test.cc
diff --git a/net/quic/core/quic_connection_test.cc b/net/quic/core/quic_connection_test.cc
index e89b5636d655df92b93e8f0140e66b72e6a32a78..55d3384cba59b3ccda88f18d1101b98aa1fddbcb 100644
--- a/net/quic/core/quic_connection_test.cc
+++ b/net/quic/core/quic_connection_test.cc
@@ -1150,8 +1150,8 @@ TEST_P(QuicConnectionTest, AllowSelfAddressChangeToMappedIpv4AddressAtServer) {
                                   kPeerAddress);
   // Cause self_address change to mapped Ipv4 address.
   QuicIpAddress host2;
-  host2.FromString(base::StringPrintf(
-      "::ffff:%s", connection_.self_address().host().ToString().c_str()));
+  host2.FromString(
+      QuicStrCat("::ffff:", connection_.self_address().host().ToString()));
   QuicSocketAddress self_address2(host2, connection_.self_address().port());
   ProcessFramePacketWithAddresses(QuicFrame(&stream_frame), self_address2,
                                   kPeerAddress);