Don't check improvement direction for return_code bisects.

scripts/slave/recipe_modules/auto_bisect/bisector_test.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.
 
 import copy
 import os
 import sys
 import unittest
 
@@ skipping 94 matching lines @@
     # Improvement down, bad > good: should not fail
     self.bisect_config['improvement_direction'] = -1
     bisector = auto_bisect.bisector.Bisector(self.dummy_api, self.bisect_config,
                                              MockRevisionClass)
     bisector.good_rev.mean_value = 10
     bisector.bad_rev.mean_value = 100
     self.assertTrue(bisector.check_improvement_direction())
     self.assertNotIn('direction of improvement', ''.join(bisector.warnings))
 
   def test_improvement_direction_return_code(self):
-    # Good revision is bad or bad revision is good, should fail.
+    # The improvement direction check doesn't apply for return code bisects.
     bisect_config = copy.deepcopy(self.bisect_config)
     bisect_config['test_type'] = 'return_code'
     bisector = auto_bisect.bisector.Bisector(self.dummy_api, bisect_config,
                                              MockRevisionClass)
     bisector.good_rev.mean_value = 1
     bisector.bad_rev.mean_value = 0
     self.assertTrue(bisector.is_return_code_mode())
-    self.assertFalse(bisector.check_improvement_direction())
-    self.assertIn('return code', ''.join(bisector.warnings))
+    self.assertTrue(bisector.check_improvement_direction())
 
   @mock.patch.object(auto_bisect.bisector.Bisector, 'significantly_different',
                      mock.MagicMock(return_value=True))
   def test_check_initial_confidence_pass(self):
     # patch bisector.significantly_different with true
     # assert both revisions have > 5 samples
     bisector = auto_bisect.bisector.Bisector(self.dummy_api, self.bisect_config,
                                              MockRevisionClass)
     bisector.good_rev.values = [3, 3, 3, 3, 3, 3]
     bisector.bad_rev.values = [3, 3, 3, 3]
@@ skipping 20 matching lines @@
     return_code_config['test_type'] = 'return_code'
     # When confidence is not required, confidence_score should not be called.
     bisector = auto_bisect.bisector.Bisector(self.dummy_api, return_code_config,
                                              MockRevisionClass)
     self.assertTrue(bisector.check_initial_confidence())
     self.assertFalse(bisector.significantly_different.called)
 
 
 if __name__ == '__main__':
   unittest.main()  # pragma: no cover