Move `gn check` into the `generate_build_files` step.

tools/mb/mb.py

 #!/usr/bin/env python
 # Copyright 2015 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.
 
 """MB - the Meta-Build wrapper around GYP and GN
 
 MB is a wrapper script for GYP and GN that can be used to generate build files
 for sets of canned configurations and analyze them.
 """
@@ skipping 336 matching lines @@
 
     if needs_clobber:
       self.RemoveDirectory(build_dir)
 
     self.MaybeMakeDirectory(build_dir)
     self.WriteFile(mb_type_path, new_mb_type)
 
   def RunGNGen(self, vals):
     path = self.args.path[0]
 
-    cmd = self.GNCmd('gen', path, vals['gn_args'])
+    cmd = self.GNCmd('gen', path, vals['gn_args'], extra_args=['--check'])
 
     swarming_targets = []
     if self.args.swarming_targets_file:
       # We need GN to generate the list of runtime dependencies for
       # the compile targets listed (one per line) in the file so
       # we can run them via swarming. We use ninja_to_gn.pyl to convert
       # the compile targets to the matching GN labels.
       contents = self.ReadFile(self.args.swarming_targets_file)
       swarming_targets = contents.splitlines()
       gn_isolate_map = ast.literal_eval(self.ReadFile(os.path.join(
@@ skipping 63 matching lines @@
             self.ToSrcRelPath('%s%s%s.isolate' % (path, os.sep, target)),
           ],
           'dir': self.chromium_src_dir,
           'version': 1,
         },
         isolate_path + 'd.gen.json',
       )
 
     return ret
 
-  def GNCmd(self, subcommand, path, gn_args=''):
+  def GNCmd(self, subcommand, path, gn_args='', extra_args=None):
     if self.platform == 'linux2':
       gn_path = os.path.join(self.chromium_src_dir, 'buildtools', 'linux64',
                              'gn')
     elif self.platform == 'darwin':
       gn_path = os.path.join(self.chromium_src_dir, 'buildtools', 'mac',
                              'gn')
     else:
       gn_path = os.path.join(self.chromium_src_dir, 'buildtools', 'win',
                              'gn.exe')
 
     cmd = [gn_path, subcommand, path]
     gn_args = gn_args.replace("$(goma_dir)", self.args.goma_dir)
     if gn_args:
       cmd.append('--args=%s' % gn_args)
+    if extra_args:
+      cmd.extend(extra_args)
     return cmd
 
   def RunGYPGen(self, vals):
     path = self.args.path[0]
 
     output_dir, gyp_config = self.ParseGYPConfigPath(path)
     if gyp_config != vals['gyp_config']:
       raise MBErr('The last component of the path (%s) must match the '
                   'GYP configuration specified in the config (%s), and '
                   'it does not.' % (gyp_config, vals['gyp_config']))
@@ skipping 366 matching lines @@
 
 if __name__ == '__main__':
   try:
     sys.exit(main(sys.argv[1:]))
   except MBErr as e:
     print(e)
     sys.exit(1)
   except KeyboardInterrupt:
     print("interrupted, exiting", stream=sys.stderr)
     sys.exit(130)