Add quic_test_loopback used for tests only.

net/quic/platform/api/quic_ip_address.h

Index: net/quic/platform/api/quic_ip_address.h
diff --git a/net/quic/platform/api/quic_ip_address.h b/net/quic/platform/api/quic_ip_address.h
index 75b9f0c7ab63b4d0050d338af529981cb76d9d78..3d3100b93201a1f95a26154ba51b6969adaf59a7 100644
--- a/net/quic/platform/api/quic_ip_address.h
+++ b/net/quic/platform/api/quic_ip_address.h
@@ -21,6 +21,8 @@ class QUIC_EXPORT_PRIVATE QuicIpAddress {
     kIPv4AddressSize = QuicIpAddressImpl::kIPv4AddressSize,
     kIPv6AddressSize = QuicIpAddressImpl::kIPv6AddressSize
   };
+
+  // TODO(fayang): Remove Loopback*() and use TestLoopback*() in tests.
   static QuicIpAddress Loopback4();
   static QuicIpAddress Loopback6();
   static QuicIpAddress Any4();