Update mb.py to support gn directories with no args.gn

tools/mb/mb_unittest.py

 #!/usr/bin/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.
 
 """Tests for mb.py."""
 
 import json
 import StringIO
 import os
@@ skipping 350 matching lines @@
                 '-c', 'gn_debug_goma',
                 '--swarming-targets-file', '/tmp/swarming_targets',
                 '//out/Default'], mbw=mbw, ret=0)
     self.assertIn('/fake_src/out/Default/base_unittests.isolate',
                   mbw.files)
     self.assertIn('/fake_src/out/Default/base_unittests.isolated.gen.json',
                   mbw.files)
 
   def test_gn_isolate(self):
     files = {
+      '/fake_src/out/Default/toolchain.ninja': "",
       '/fake_src/testing/buildbot/gn_isolate_map.pyl': (
           "{'base_unittests': {"
           "  'label': '//base:base_unittests',"
           "  'type': 'raw',"
           "  'args': [],"
           "}}\n"
       ),
       '/fake_src/out/Default/base_unittests.runtime_deps': (
           "base_unittests\n"
       ),
@@ skipping 87 matching lines @@
     self.check(['validate'], mbw=mbw, ret=0)
 
   def test_bad_validate(self):
     mbw = self.fake_mbw()
     mbw.files[mbw.default_config] = TEST_BAD_CONFIG
     self.check(['validate'], mbw=mbw, ret=1)
 
 
 if __name__ == '__main__':
   unittest.main()