Add quic_test_loopback used for tests only.

net/quic/platform/impl/quic_test_loopback_impl.h

+// 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.
+
+#ifndef NET_QUIC_PLATFORM_IMPL_QUIC_TEST_LOOPBACK_IMPL_H_
+#define NET_QUIC_PLATFORM_IMPL_QUIC_TEST_LOOPBACK_IMPL_H_
+
+#include "net/quic/platform/api/quic_ip_address.h"
+
+namespace net {
+
+// Returns the address family IPv4 used to run test under.
+IpAddressFamily AddressFamilyUnderTestImpl();
+
+// Returns an IPv4 loopback address.
+QuicIpAddress TestLoopback4Impl();
+
+// Returns the only IPv6 loopback address.
+QuicIpAddress TestLoopback6Impl();
+
+// Returns an IPv4 loopback address.
+QuicIpAddress TestLoopbackImpl();
+
+// Returns an indexed IPv4 loopback address.
+QuicIpAddress TestLoopbackImpl(int index);
+
+}  // namespace net
+
+#endif  // NET_QUIC_PLATFORM_IMPL_QUIC_TEST_LOOPBACK_IMPL_H_