Don't check improvement direction for return_code bisects.

scripts/slave/recipe_modules/auto_bisect/bisector.py

 # 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 json
 import re
 import time
 import urllib
 
 from . import depot_config
@@ skipping 428 matching lines @@
       if i < len(self.revisions) - 1:
         self.revisions[i].next_revision = self.revisions[i + 1]
 
   def check_improvement_direction(self):  # pragma: no cover
     """Verifies that the change from 'good' to 'bad' is in the right direction.
 
     The change between the test results obtained for the given 'good' and
     'bad' revisions is expected to be considered a regression. The
     `improvement_direction` attribute is positive if a larger number is
     considered better, and negative if a smaller number is considered better.
+
+    Returns:
+      True if the check passes (i.e. no problem), False if the change is not
+      a regression according to the improvement direction.
     """
     good = self.good_rev.mean_value
     bad = self.bad_rev.mean_value
 
     if self.is_return_code_mode():
-      if good == 1 or bad == 0:
-        self._set_failed_return_code_direction_results()
-        return False
       return True
 
     direction = self.improvement_direction
     if direction is None:
       return True
     if (bad > good and direction > 0) or (bad < good and direction < 0):
       self._set_failed_direction_results()
       return False
     return True
 
@@ skipping 376 matching lines @@
         })
     return revision_rows
 
   def _get_build_url(self):
     properties = self.api.m.properties
     bot_url = properties.get('buildbotURL',
                              'http://build.chromium.org/p/chromium/')
     builder_name = urllib.quote(properties.get('buildername', ''))
     builder_number = str(properties.get('buildnumber', ''))
     return '%sbuilders/%s/builds/%s' % (bot_url, builder_name, builder_number)