Add QuicStrCat.

net/quic/core/quic_session_test.cc

Index: net/quic/core/quic_session_test.cc
diff --git a/net/quic/core/quic_session_test.cc b/net/quic/core/quic_session_test.cc
index 660e2d88a598503dd4d5b1874d9caa8515126c38..19d35de4e034b52eb838c5b13f9b145a7c9f3f1e 100644
--- a/net/quic/core/quic_session_test.cc
+++ b/net/quic/core/quic_session_test.cc
@@ -921,9 +921,8 @@ TEST_P(QuicSessionTestServer,
   while (!headers_stream->flow_controller()->IsBlocked() && stream_id < 2000) {
     EXPECT_FALSE(session_.IsConnectionFlowControlBlocked());
     EXPECT_FALSE(session_.IsStreamFlowControlBlocked());
-    headers["header"] = base::Uint64ToString(base::RandUint64()) +
-                        base::Uint64ToString(base::RandUint64()) +
-                        base::Uint64ToString(base::RandUint64());
+    headers["header"] = QuicStrCat("", base::RandUint64(), base::RandUint64(),
+                                   base::RandUint64());
     headers_stream->WriteHeaders(stream_id, headers.Clone(), true, 0, nullptr);
     stream_id += 2;
   }