Implement target visibility in GN

tools/gn/gn.gyp

 {
   'variables': {
     'chromium_code': 1,
   },
   'targets': [
     {
       'target_name': 'gn_lib',
       'type': 'static_library',
       'dependencies': [
         '../../base/base.gyp:base',
@@ skipping 129 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',
       ],
     },
     {
       'target_name': 'gn',
       'type': 'executable',
       'sources': [
         'gn_main.cc',
       ],
       'dependencies': [
         'gn_lib',
@@ skipping 25 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',
       ],
       'dependencies': [
         'gn_lib',
         '../../base/base.gyp:run_all_unittests',
         '../../base/base.gyp:test_support_base',
         '../../testing/gtest.gyp:gtest',
       ],
     },
     {
       'target_name': 'generate_test_gn_data',
       'type': 'executable',
       'sources': [
         'generate_test_gn_data.cc',
       ],
       'dependencies': [
         '../../base/base.gyp:base',
       ],
     }
   ],
 }