Add quic_test_loopback used for tests only.

net/quic/platform/api/quic_test_loopback.h

Index: net/quic/platform/api/quic_test_loopback.h
diff --git a/net/quic/platform/api/quic_test_loopback.h b/net/quic/platform/api/quic_test_loopback.h
new file mode 100644
index 0000000000000000000000000000000000000000..db7d2c085c03cef3d8349aa7e017ab8411bce114
--- /dev/null
+++ b/net/quic/platform/api/quic_test_loopback.h
@@ -0,0 +1,32 @@
+// 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_API_QUIC_TEST_LOOPBACK_H_
+#define NET_QUIC_PLATFORM_API_QUIC_TEST_LOOPBACK_H_
+
+#include "net/quic/platform/impl/quic_test_loopback_impl.h"
+
+namespace net {
+
+// Returns the address family (IPv4 or IPv6) used to run test under.
+IpAddressFamily AddressFamilyUnderTest();
+
+// Returns an IPv4 loopback address.
+QuicIpAddress TestLoopback4();
+
+// Returns the only IPv6 loopback address.
+QuicIpAddress TestLoopback6();
+
+// Returns an appropriate IPv4/Ipv6 loopback address based upon whether the
+// test's environment.
+QuicIpAddress TestLoopback();
+
+// If address family under test is IPv4, returns an indexed IPv4 loopback
+// address. If address family under test is IPv6, the address returned is
+// platform-dependent.
+QuicIpAddress TestLoopback(int index);
+
+}  // namespace net
+
+#endif  // NET_QUIC_PLATFORM_API_QUIC_TEST_LOOPBACK_H_