CQ: do not force HEAD revision for chromium_trybot builders

commit-queue/projects.py

 # Copyright (c) 2012 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.
 """Define the supported projects."""
 
 import json
 import logging
 import os
 import re
 import sys
@@ skipping 441 matching lines @@
               builder_name=triggered,
               trigger_name=builder,
               prereq_builder=prereq_builder,
               prereq_tests=prereq_tests,
               steps=builder_swarm_enabled_tests,
               trigger_bot_steps=regular_tests,
               use_triggered_bot=False))
 
     # Experimental recipe-based Chromium trybots. To avoid possible capacity
     # problems, only enable for a small percentage of try runs.
-    # Disable as emergency measure - crbug.com/334681
+    #
+    # Note how we pass revision=None . In presence of safesync_url
+    # this makes a difference: no revision uses safesync_url,
+    # while HEAD ignores it and always fetches latest revision.
     verifiers.append(
         trigger_experimental_try_job.TriggerExperimentalTryJobVerifier(
             context_obj,
             percentage=0.25,
+            revision=None,
             try_job_description={
                 'linux_chromium_rel': ['defaulttests'],
                 'mac_chromium_rel': ['defaulttests'],
             }))
 
   verifiers.append(try_job_on_rietveld.TryRunnerRietveld(
       context_obj,
       'http://build.chromium.org/p/tryserver.chromium/',
       user,
       step_verifiers,
@@ skipping 286 matching lines @@
   """List the projects that can be managed by the commit queue."""
   return sorted(
       x[5:] for x in dir(sys.modules[__name__]) if x.startswith('_gen_'))
 
 
 def load_project(project, user, root_dir, rietveld_obj, no_try):
   """Loads the specified project."""
   assert os.path.isabs(root_dir)
   return getattr(sys.modules[__name__], '_gen_' + project)(
       user, root_dir, rietveld_obj, no_try)