Add QuicStrCat.

net/quic/core/quic_server_id.cc

Index: net/quic/core/quic_server_id.cc
diff --git a/net/quic/core/quic_server_id.cc b/net/quic/core/quic_server_id.cc
index ecc8d0e40fd952383a9351412160093773ac44f9..493fccd72f37fbb99e2967ae3e293c99e3ae8622 100644
--- a/net/quic/core/quic_server_id.cc
+++ b/net/quic/core/quic_server_id.cc
@@ -9,6 +9,7 @@
 #include "base/logging.h"
 #include "net/base/host_port_pair.h"
 #include "net/base/port_util.h"
+#include "net/quic/platform/api/quic_str_cat.h"
 #include "url/gurl.h"
 
 using std::string;
@@ -52,8 +53,8 @@ QuicServerId QuicServerId::FromString(const std::string& str) {
 }
 
 string QuicServerId::ToString() const {
-  return "https://" + host_port_pair_.ToString() +
-         (privacy_mode_ == PRIVACY_MODE_ENABLED ? "/private" : "");
+  return QuicStrCat("https://", host_port_pair_.ToString(),
+                    (privacy_mode_ == PRIVACY_MODE_ENABLED ? "/private" : ""));
 }
 
 }  // namespace net