Add quic_test_loopback used for tests only.

net/quic/platform/api/quic_test_loopback.cc

+// 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