GN only writes files if they've changed

tools/gn/gn.gyp

 {
   'variables': {
     'chromium_code': 1,
   },
   'targets': [
     {
       'target_name': 'gn_lib',
       'type': 'static_library',
       'dependencies': [
         '../../base/base.gyp:base',
@@ skipping 167 matching lines @@
         'builder_unittest.cc',
         'c_include_iterator_unittest.cc',
         'config_values_extractors_unittest.cc',
         'escape_unittest.cc',
         'filesystem_utils_unittest.cc',
         'file_template_unittest.cc',
         'function_foreach_unittest.cc',
         'function_get_label_info_unittest.cc',
         'function_get_target_outputs_unittest.cc',
         'function_rebase_path_unittest.cc',
+        'function_write_file_unittest.cc',
         'functions_unittest.cc',
         'header_checker_unittest.cc',
         'input_conversion_unittest.cc',
         'label_unittest.cc',
         'loader_unittest.cc',
         'ninja_action_target_writer_unittest.cc',
         'ninja_binary_target_writer_unittest.cc',
         'ninja_copy_target_writer_unittest.cc',
         'ninja_helper_unittest.cc',
         'ninja_target_writer_unittest.cc',
@@ skipping 26 matching lines @@
       'type': 'executable',
       'sources': [
         'generate_test_gn_data.cc',
       ],
       'dependencies': [
         '../../base/base.gyp:base',
       ],
     }
   ],
 }