Add optional public header checking to GN build

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",
     "action_values.cc",
     "action_values.h",
     "args.cc",
     "args.h",
     "binary_target_generator.cc",
     "binary_target_generator.h",
     "build_settings.cc",
     "build_settings.h",
     "builder.cc",
     "builder.h",
     "builder_record.cc",
     "builder_record.h",
+    "c_include_iterator.cc",
+    "c_include_iterator.h",
     "command_args.cc",
+    "command_check.cc",
     "command_desc.cc",
     "command_gen.cc",
     "command_help.cc",
     "command_refs.cc",
     "commands.cc",
     "commands.h",
     "config.cc",
     "config.h",
     "config_values.cc",
     "config_values.h",
@@ skipping 18 matching lines @@
     "function_process_file_template.cc",
     "function_read_file.cc",
     "function_rebase_path.cc",
     "function_set_default_toolchain.cc",
     "function_set_defaults.cc",
     "function_template.cc",
     "function_toolchain.cc",
     "function_write_file.cc",
     "group_target_generator.cc",
     "group_target_generator.h",
+    "header_checker.cc",
+    "header_checker.h",
     "import_manager.cc",
     "import_manager.h",
     "input_conversion.cc",
     "input_conversion.h",
     "input_file.cc",
     "input_file.h",
     "input_file_manager.cc",
     "input_file_manager.h",
     "item.cc",
     "item.h",
@@ skipping 88 matching lines @@
 
   deps = [
     ":gn_lib",
     "//build/util:last_change",
   ]
 }
 
 test("gn_unittests") {
   sources = [
     "builder_unittest.cc",
+    "c_include_iterator_unittest.cc",
     "escape_unittest.cc",
     "filesystem_utils_unittest.cc",
     "file_template_unittest.cc",
     "function_rebase_path_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",
     "operators_unittest.cc",
     "parse_tree_unittest.cc",
     "parser_unittest.cc",
@@ skipping 15 matching lines @@
     "//base/test:run_all_unittests",
     "//base/test:test_support_base",
     "//testing/gtest",
   ]
 }
 
 executable("generate_test_gn_data") {
   sources = [ "generate_test_gn_data.cc" ]
   deps = [ "//base" ]
 }