Don't check improvement direction for return_code bisects.

scripts/slave/recipe_modules/auto_bisect/bisector.py

Index: scripts/slave/recipe_modules/auto_bisect/bisector.py
diff --git a/scripts/slave/recipe_modules/auto_bisect/bisector.py b/scripts/slave/recipe_modules/auto_bisect/bisector.py
index e6caa9b47da7853f5ece3715996a20bd77c01c86..e79e132973747073f4179afafcbf3b3b79fca039 100644
--- a/scripts/slave/recipe_modules/auto_bisect/bisector.py
+++ b/scripts/slave/recipe_modules/auto_bisect/bisector.py
@@ -446,14 +446,15 @@ class Bisector(object):
     '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