Gsutil ls misses should not be treated as failures by bisect.

scripts/slave/recipe_modules/auto_bisect/api.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.
 
 """API for the bisect recipe module.
 
 This API is meant to enable the bisect recipe to bisect any chromium-supported
 platform for any test that can be run via buildbot, perf or otherwise.
 """
 
@@ skipping 81 matching lines @@
 
   def set_svn_repo(self, svn_repo_url):  # pragma: no cover
     """Sets SVN repo url for triggering build jobs."""
     self.svn_repo_url = svn_repo_url
 
   def gsutil_file_exists(self, path):
     """Returns True if a file exists at the given GS path."""
     try:
       self.m.gsutil(['ls', path])
     except self.m.step.StepFailure:  # pragma: no cover
+      # A step failure here simply means that the file does not exist, and
+      # should not be treated as an error.
+      self.m.step.active_result.presentation.status = self.m.step.SUCCESS
       return False
     return True
 
   def query_revision_info(self, revision):
     """Gathers information on a particular revision.
 
     Args:
       revision (str): A git commit hash.
 
     Returns:
@@ skipping 276 matching lines @@
             affected_files, update_step, self.bot_db)
     finally:
       if api.chromium.c.TARGET_PLATFORM == 'android':
         if self.internal_bisect:  # pragma: no cover
           api.chromium_android.init_and_sync(
               gclient_config=api.chromium_android.c.internal_dir_name,
               use_bot_update=True)
         else:
           api.bot_update.ensure_checkout()
         api.chromium_android.common_tests_final_steps()