Added method for loading of the gatekeeper_trees.json config.

scripts/slave/unittests/gatekeeper_ng_test.py

 #!/usr/bin/env python
 # Copyright 2013 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 gatekeeper_ng.py.
 
 This is a basic check that gatekeeper_ng.py can properly interpret builds and
 close the tree.
 
@@ skipping 2714 matching lines @@
                                 {'squirrels': ['yay']})
     with self.assertRaises(AssertionError):
       self.call_gatekeeper()
 
   # Check that the checked in gatekeeper.json is valid.
   def testCheckedInConfigIsValid(self):
     sys.argv.extend(['--verify'])
     self.call_gatekeeper(
         json=os.path.join(SCRIPT_DIR, os.pardir, 'gatekeeper.json'))
 
+  def testCheckedInTreeConfigIsValid(self): # pylint: disable=R0201
+    tree_config = os.path.join(SCRIPT_DIR, os.pardir, 'gatekeeper_trees.json')
+    gatekeeper_ng_config.load_gatekeeper_tree_config(tree_config)
+
 
 if __name__ == '__main__':
   with utils.print_coverage(include=[__file__]):
     unittest.main()