Add canned presubmit check for GN formatting

tests/presubmit_unittest.py

 #!/usr/bin/env python
 # Copyright (c) 2012 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.
 
 """Unit tests for presubmit_support.py and presubmit_canned_checks.py."""
 
 # pylint: disable=E1101,E1103
 
 import functools
@@ skipping 1860 matching lines @@
       'CheckChangeHasTestField',
       'CheckChangeLintsClean',
       'CheckChangeSvnEolStyle',
       'CheckChangeWasUploaded',
       'CheckDoNotSubmit',
       'CheckDoNotSubmitInDescription', 'CheckDoNotSubmitInFiles',
       'CheckLongLines', 'CheckTreeIsOpen', 'PanProjectChecks',
       'CheckLicense',
       'CheckOwners',
       'CheckPatchFormatted',
+      'CheckGNFormatted',
       'CheckRietveldTryJobExecution',
       'CheckSingletonInHeaders',
       'CheckSvnModifiedDirectories',
       'CheckSvnForCommonMimeTypes', 'CheckSvnProperty',
       'RunPythonUnitTests', 'RunPylint',
       'RunUnitTests', 'RunUnitTestsInDirectory',
       'GetPythonUnitTests', 'GetPylint',
       'GetUnitTests', 'GetUnitTestsInDirectory', 'GetUnitTestsRecursively',
     ]
     # If this test fails, you should add the relevant test.
@@ skipping 1028 matching lines @@
         owners_check=False)
     self.assertEqual(1, len(results))
     self.assertEqual(
         'Found line ending with white spaces in:', results[0]._message)
     self.checkstdout('')
 
 
 if __name__ == '__main__':
   import unittest
   unittest.main()