Switch the default `git try` / `gcl try` bots to _rel.

PRESUBMIT.py

 # Copyright (c) 2011 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 Chromium.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into gcl.
 """
 
@@ skipping 299 matching lines @@
   results.extend(_CommonChecks(input_api, output_api))
   # TODO(thestig) temporarily disabled, doesn't work in third_party/
   #results.extend(input_api.canned_checks.CheckSvnModifiedDirectories(
   #    input_api, output_api, sources))
   # Make sure the tree is 'open'.
   results.extend(input_api.canned_checks.CheckTreeIsOpen(
       input_api,
       output_api,
       json_url='http://chromium-status.appspot.com/current?format=json'))
   results.extend(input_api.canned_checks.CheckRietveldTryJobExecution(input_api,
-      output_api, 'http://codereview.chromium.org', ('win', 'linux', 'mac'),
-      'tryserver@chromium.org'))
+      output_api, 'http://codereview.chromium.org',
+      ('win_rel', 'linux_rel', 'mac_rel'), 'tryserver@chromium.org'))
 
   results.extend(input_api.canned_checks.CheckChangeHasBugField(
       input_api, output_api))
   results.extend(input_api.canned_checks.CheckChangeHasTestField(
       input_api, output_api))
   results.extend(input_api.canned_checks.CheckChangeHasDescription(
       input_api, output_api))
   results.extend(_CheckSubversionConfig(input_api, output_api))
   return results
 
 
 def GetPreferredTrySlaves(project, change):
   only_objc_files = all(
       f.LocalPath().endswith(('.mm', '.m')) for f in change.AffectedFiles())
   if only_objc_files:
-    return ['mac']
-  return ['win', 'linux', 'mac']
+    return ['mac_rel']
+  return ['win_rel', 'linux_rel', 'mac_rel']