Add optional public header checking to GN build

tools/gn/BUILD.gn

Index: tools/gn/BUILD.gn
diff --git a/tools/gn/BUILD.gn b/tools/gn/BUILD.gn
index e176b92127e5f062ad44b3604eee0d046560cc66..44305f7a5ca29a0458f21e59f3ff3fa499ee00db 100644
--- a/tools/gn/BUILD.gn
+++ b/tools/gn/BUILD.gn
@@ -20,7 +20,10 @@ static_library("gn_lib") {
     "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",
@@ -59,6 +62,8 @@ static_library("gn_lib") {
     "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",
@@ -167,11 +172,13 @@ executable("gn") {
 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",