Implement custom extensions.

This change mirrors upstream's custom extension API because we have some
internal users that depend on it.

Change-Id: I408e442de0a55df7b05c872c953ff048cd406513
Reviewed-on: https://boringssl-review.googlesource.com/5471
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/test/test_config.cc b/ssl/test/test_config.cc
index fef51ed..d873d8e 100644
--- a/ssl/test/test_config.cc
+++ b/ssl/test/test_config.cc
@@ -84,6 +84,12 @@
   { "-expect-no-session", &TestConfig::expect_no_session },
   { "-use-ticket-callback", &TestConfig::use_ticket_callback },
   { "-renew-ticket", &TestConfig::renew_ticket },
+  { "-enable-client-custom-extension",
+    &TestConfig::enable_client_custom_extension },
+  { "-enable-server-custom-extension",
+    &TestConfig::enable_server_custom_extension },
+  { "-custom-extension-skip", &TestConfig::custom_extension_skip },
+  { "-custom-extension-fail-add", &TestConfig::custom_extension_fail_add },
 };
 
 const Flag<std::string> kStringFlags[] = {