Fix the PRESUBMIT.py for depot_tools.

PRESUBMIT.py

 #!/usr/bin/python
 # Copyright (c) 2009 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.
 
 """Top-level presubmit script for depot tools.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts for
 details on the presubmit API built into gcl.
 """
 
+UNIT_TESTS = [
+  'tests.gcl_unittest',
+  'tests.gclient_test',
+  'tests.presubmit_unittest',
+  'tests.revert_unittest',
+  'tests.trychange_unittest',
+]
 
 def CheckChangeOnUpload(input_api, output_api):
-  return RunUnitTests(input_api, output_api)
+  output = []
+  output.extend(input_api.canned_checks.RunPythonUnitTests(input_api,
+                                                           output_api,
+                                                           UNIT_TESTS))
+  return output
 
 
 def CheckChangeOnCommit(input_api, output_api):
-  unit_tests = [
-    'tests.gcl_unittest',
-    'tests.gclient_test',
-    'tests.presubmit_unittest',
-    'tests.revert_unittest',
-    'tests.trychange_unittest',
-  ]
   output = []
   output.extend(input_api.canned_checks.RunPythonUnitTests(input_api,
                                                            output_api,
-                                                           unit_tests))
+                                                           UNIT_TESTS))
   output.extend(input_api.canned_checks.CheckDoNotSubmit(input_api,
                                                          output_api))
   return output