Allow GN configs to have sub-configs.

tools/gn/BUILD.gn

 # Copyright (c) 2013 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.
 
 import("//testing/test.gni")
 
 defines = [ "GN_BUILD" ]
 
 static_library("gn_lib") {
   configs += [ "//build/config:precompiled_headers" ]
@@ skipping 231 matching lines @@
   ]
 }
 
 test("gn_unittests") {
   sources = [
     "action_target_generator_unittest.cc",
     "args_unittest.cc",
     "builder_unittest.cc",
     "c_include_iterator_unittest.cc",
     "command_format_unittest.cc",
+    "config_unittest.cc",
     "config_values_extractors_unittest.cc",
     "escape_unittest.cc",
     "exec_process_unittest.cc",
     "filesystem_utils_unittest.cc",
     "function_foreach_unittest.cc",
     "function_forward_variables_from_unittest.cc",
     "function_get_label_info_unittest.cc",
     "function_get_path_info_unittest.cc",
     "function_get_target_outputs_unittest.cc",
     "function_process_file_template_unittest.cc",
@@ skipping 51 matching lines @@
 
 executable("generate_test_gn_data") {
   sources = [
     "generate_test_gn_data.cc",
   ]
   deps = [
     "//base",
     "//build/config/sanitizers:deps",
   ]
 }