Fix xvfb for blink trybot.

scripts/slave/recipes/blink_trybot.py

 # Copyright 2013 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.
 
 DEPS = [
   'chromium',
   'gclient',
   'json',
   'path',
   'platform',
@@ skipping 37 matching lines @@
 
         if r.unexpected_flakes or r.unexpected_failures:
           p.status = 'WARNING'
         else:
           p.status = 'SUCCESS'
 
       yield api.chromium.runtests(self.layout_test_wrapper,
                                   args,
                                   name=self._step_name(suffix),
                                   can_fail_build=False,
+                                  xvfb=True,
                                   followup_fn=followup_fn)
 
       if suffix == 'with patch':
         buildername = api.properties['buildername']
         buildnumber = api.properties['buildnumber']
         def archive_webkit_tests_results_followup(step_result):
           base = (
               "https://storage.googleapis.com/chromium-layout-test-archives/%s/%s" %
               (buildername, buildnumber))
 
@@ skipping 117 matching lines @@
     api.test('bad_revert_bails') +
     props() +
     api.step_data('gclient revert', retcode=1)
   )
 
   yield (
     api.test('bad_sync_bails') +
     props() +
     api.step_data('gclient sync', retcode=1)
   )