Allow GN configs to have sub-configs.

tools/gn/gn.gyp

 {
   'variables': {
     'chromium_code': 1,
   },
   'targets': [
     {
       'target_name': 'gn_lib',
       'type': 'static_library',
       'dependencies': [
         '../../base/base.gyp:base',
@@ skipping 191 matching lines @@
       ],
     },
     {
       'target_name': 'gn_unittests',
       'type': '<(gtest_target_type)',
       'sources': [
         'action_target_generator_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 63 matching lines @@
           'dependencies': [
             'gn_unittests',
           ],
           'includes': [ '../../build/isolate.gypi' ],
           'sources': [ 'gn_unittests.isolate' ],
         },
       ],
     }],
   ],
 }