CQ: do not force HEAD revision for chromium_trybot builders

commit-queue/verification/trigger_experimental_try_job.py

Index: commit-queue/verification/trigger_experimental_try_job.py
===================================================================
--- commit-queue/verification/trigger_experimental_try_job.py	(revision 247015)
+++ commit-queue/verification/trigger_experimental_try_job.py	(working copy)
@@ -13,17 +13,18 @@
 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):