GN only writes files if they've changed

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.
 
 defines = [ "GN_BUILD" ]
 
 static_library("gn_lib") {
   sources = [
     "action_target_generator.cc",
     "action_target_generator.h",
@@ skipping 166 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 20 matching lines @@
     "//base/test:run_all_unittests",
     "//base/test:test_support",
     "//testing/gtest",
   ]
 }
 
 executable("generate_test_gn_data") {
   sources = [ "generate_test_gn_data.cc" ]
   deps = [ "//base" ]
 }