Implement target visibility in GN

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 125 matching lines @@
     "toolchain.cc",
     "toolchain.h",
     "trace.cc",
     "trace.h",
     "value.cc",
     "value.h",
     "value_extractors.cc",
     "value_extractors.h",
     "variables.cc",
     "variables.h",
+    "visibility.cc",
+    "visibility.h",
   ]
 
   deps = [
     "//base",
     "//base/third_party/dynamic_annotations",
   ]
 }
 
 executable("gn") {
   sources = [
@@ skipping 28 matching lines @@
     "pattern_unittest.cc",
     "scope_per_file_provider_unittest.cc",
     "scope_unittest.cc",
     "source_dir_unittest.cc",
     "string_utils_unittest.cc",
     "target_generator_unittest.cc",
     "target_unittest.cc",
     "test_with_scope.cc",
     "test_with_scope.h",
     "tokenizer_unittest.cc",
+    "visibility_unittest.cc",
   ]
   deps = [
     ":gn_lib",
     "//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" ]
 }