Add the concept of a source set to GN.

tools/gn/gn.gyp

 {
   'variables': {
     'chromium_code': 1,
   },
   'targets': [
     {
       'target_name': 'gn_lib',
       'type': 'static_library',
       'dependencies': [
         '../../base/base.gyp:base',
@@ skipping 153 matching lines @@
     {
       'target_name': 'gn_unittests',
       'type': '<(gtest_target_type)',
       'sources': [
         'escape_unittest.cc',
         'file_template_unittest.cc',
         'filesystem_utils_unittest.cc',
         'function_rebase_path_unittest.cc',
         'input_conversion_unittest.cc',
         'label_unittest.cc',
+        'ninja_binary_target_writer_unittest.cc',
         'ninja_helper_unittest.cc',
         'ninja_copy_target_writer_unittest.cc',
         'ninja_script_target_writer_unittest.cc',
         'parser_unittest.cc',
         'path_output_unittest.cc',
         'pattern_unittest.cc',
         'scope_per_file_provider_unittest.cc',
         'source_dir_unittest.cc',
         'string_utils_unittest.cc',
         'target_generator_unittest.cc',
@@ skipping 15 matching lines @@
       'type': 'executable',
       'sources': [
         'generate_test_gn_data.cc',
       ],
       'dependencies': [
         '../../base/base.gyp:base',
       ],
     }
   ],
 }