CQ: do not force HEAD revision for chromium_trybot builders

commit-queue/verification/trigger_experimental_try_job.py

 # coding=utf8
 # Copyright 2014 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.
 """Triggers experimental try jobs on Rietveld without monitoring the results."""
 
 import random
 import urllib2
 
 from verification import base
 
 
 class TriggerExperimentalTryJobVerifier(base.Verifier):
   name = 'trigger experimental try job'
 
-  def __init__(self, context, percentage, try_job_description):
+  def __init__(self, context, percentage, revision, try_job_description):
     super(TriggerExperimentalTryJobVerifier, self).__init__()
     self.context = context
     self.percentage = percentage
+    self.revision = revision
     self.try_job_description = try_job_description
 
   def verify(self, pending):
     if random.random() < self.percentage:
       try:
         self.context.rietveld.trigger_try_jobs(
-            pending.issue, pending.patchset, 'CQ', False, 'HEAD',
+            pending.issue, pending.patchset, 'CQ', False, self.revision,
             self.try_job_description)
       except urllib2.HTTPError as e:
         if e.code not in (400, 500, 503):
           raise
 
     # Always succeed - experimental try jobs are not stable enough
     # to block CQ on them, but triggering builds allows developers
     # to get enough real-world traffic to get them to stable state.
     pending.verifications[self.name] = base.SimpleStatus(base.SUCCEEDED)
 
   def update_status(self, queue):
     pass