GN: add default target.

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 122 matching lines @@
     # Use the configured goma directory.
     local_goma_dir = _get_gn_arg_value(out_dir, 'goma_dir')
     print 'Ensuring goma (in %s) started ...' % local_goma_dir
     command = ['python',
                os.path.join(local_goma_dir, 'goma_ctl.py'),
                'ensure_start']
     exit_code = subprocess.call(command)
     if exit_code:
       return exit_code
 
-    return subprocess.call(['ninja', '-j', '1000', '-l', '100', '-C', out_dir,
-                            'root'])
+    return subprocess.call(['ninja', '-j', '1000', '-l', '100', '-C', out_dir])
   else:
-    return subprocess.call(['ninja', '-C', out_dir, 'root'])
+    return subprocess.call(['ninja', '-C', out_dir])
 
 
 def _run_tests(config, test_types):
   """Runs the tests of the given type(s) for the given config."""
 
   assert isinstance(test_types, list)
   config = deepcopy(config)
   config.values['test_types'] = test_types
 
   test_list = GetTestList(config)
@@ skipping 108 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())