Add quic_test_loopback used for tests only.

net/quic/platform/impl/quic_test_loopback_impl.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/impl/quic_test_loopback_impl.h"
+
+namespace net {
+
+IpAddressFamily AddressFamilyUnderTestImpl() {
+  return IpAddressFamily::IP_V4;
+}
+
+QuicIpAddress TestLoopback4Impl() {
+  return QuicIpAddress(QuicIpAddressImpl(IPAddress::IPv4Localhost()));
+}
+
+QuicIpAddress TestLoopback6Impl() {
+  return QuicIpAddress(QuicIpAddressImpl(IPAddress::IPv6Localhost()));
+}
+
+QuicIpAddress TestLoopbackImpl() {
+  return QuicIpAddress(QuicIpAddressImpl(IPAddress::IPv4Localhost()));
+}
+
+QuicIpAddress TestLoopbackImpl(int index) {
+  const uint8_t kLocalhostIPv4[] = {127, 0, 0, index};
+  return QuicIpAddress(QuicIpAddressImpl(IPAddress(kLocalhostIPv4)));
+}
+
+}  // namespace net