Add quic_test_loopback used for tests only.

net/quic/platform/api/quic_test_loopback.cc

Index: net/quic/platform/api/quic_test_loopback.cc
diff --git a/net/quic/platform/api/quic_test_loopback.cc b/net/quic/platform/api/quic_test_loopback.cc
new file mode 100644
index 0000000000000000000000000000000000000000..ea240604897c76b5657862ce2ce86622c88bd031
--- /dev/null
+++ b/net/quic/platform/api/quic_test_loopback.cc
@@ -0,0 +1,29 @@
+// Copyright (c) 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "net/quic/platform/api/quic_test_loopback.h"
+
+namespace net {
+
+IpAddressFamily AddressFamilyUnderTest() {
+  return AddressFamilyUnderTestImpl();
+}
+
+QuicIpAddress TestLoopback4() {
+  return TestLoopback4Impl();
+}
+
+QuicIpAddress TestLoopback6() {
+  return TestLoopback6Impl();
+}
+
+QuicIpAddress TestLoopback() {
+  return TestLoopbackImpl();
+}
+
+QuicIpAddress TestLoopback(int index) {
+  return TestLoopbackImpl(index);
+}
+
+}  // namespace net