Use gn check_targets and gn gen --check instead of presubmit

mojo/tools/mojob.py

 #!/usr/bin/env python
 # Copyright 2014 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.
 
 # This a simple script to make building/testing Mojo components easier.
 
 import argparse
 from copy import deepcopy
 import os
@@ skipping 60 matching lines @@
   paths = Paths(config)
   return paths.SrcRelPath(paths.build_dir)
 
 
 def sync(config):
   # pylint: disable=W0613
   return subprocess.call(['gclient', 'sync'])
 
 
 def gn(config):
-  command = ['gn', 'gen']
+  command = ['gn', 'gen', '--check']
 
   gn_args = []
   gn_args.append('is_debug=' + ('true' if config.is_debug else 'false'))
   gn_args.append('is_asan=' + ('true' if config.sanitizer ==
                                Config.SANITIZER_ASAN else 'false'))
   if config.is_clang is not None:
     gn_args.append('is_clang=' + ('true' if config.is_clang else 'false'))
   else:
     gn_args.append('is_clang=' + ('true' if config.target_os not in
                                   (Config.OS_ANDROID, Config.OS_WINDOWS)
@@ skipping 172 matching lines @@
       help='Run Dart unit tests (does not build).')
   darttest_parser.set_defaults(func=darttest)
 
   args = parser.parse_args()
   config = _args_to_config(args)
   return args.func(config)
 
 
 if __name__ == '__main__':
   sys.exit(main())