Fix xvfb for blink trybot.

scripts/slave/recipes/blink_trybot.expected/mac_passFirst_release_git.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "config",
       "--spec",
       "cache_dir = '[ROOT]/git_cache'\nsolutions = [{'custom_vars': {'googlecode_url': 'svn://svn-mirror.golo.chromium.org/%s', 'nacl_trunk': 'svn://svn-mirror.golo.chromium.org/native_client/trunk', 'sourceforge_url': 'svn://svn-mirror.golo.chromium.org/%(repo)s', 'webkit_revision': 'HEAD', 'webkit_trunk': 'https://chromium.googlesource.com/chromium/blink.git'}, 'deps_file': '.DEPS.git', 'managed': True, 'name': 'src', 'url': 'https://chromium.googlesource.com/chromium/src.git'}, {'custom_deps': {'src/chrome/test/data/firefox2_profile/searchplugins': None, 'src/chrome/test/data/firefox2_searchplugins': None, 'src/chrome/test/data/firefox3_profile/searchplugins': None, 'src/chrome/test/data/firefox3_searchplugins': None, 'src/chrome/test/data/osdd': None, 'src/chrome/test/data/perf/frame_rate/private': None, 'src/chrome/test/data/plugin': None, 'src/chrome/test/data/ssl/certs': None, 'src/data/autodiscovery': None, 'src/data/esctf': None, 'src/data/mach_ports': None, 'src/data/memory_test': None, 'src/data/mozilla_js_tests': None, 'src/data/page_cycler': None, 'src/data/selenium_core': None, 'src/data/tab_switching': None, 'src/tools/grit/grit/test/data': None}, 'deps_file': '.DEPS.git', 'managed': True, 'name': 'src-internal', 'url': 'https://chrome-internal.googlesource.com/chrome/src-internal.git'}]"
     ],
@@ skipping 250 matching lines @@
     "always_run": true,
     "can_fail_build": false,
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/runtest.py",
       "--target",
       "Release",
       "--build-dir",
       "[SLAVE_BUILD]/src/out",
-      "--no-xvfb",
+      "--xvfb",
       "--factory-properties",
       "{\"GIT_MODE\": true, \"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"branch\": \"\", \"build_config\": \"Release\", \"buildername\": \"TestBuilder\", \"buildnumber\": 571, \"config_name\": \"blink\", \"issue\": 12853011, \"mastername\": \"chromium.testing.master\", \"patchset\": 1, \"project\": \"chrome\", \"recipe\": \"blink_trybot\", \"repository\": \"\", \"requester\": \"commit-bot@chromium.org\", \"revision\": \"HEAD\", \"rietveld\": \"https://codereview.chromium.org\", \"root\": \"src/third_party/WebKit\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}",
       "--build-properties",
       "{\"GIT_MODE\": true, \"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"branch\": \"\", \"build_config\": \"Release\", \"buildername\": \"TestBuilder\", \"buildnumber\": 571, \"config_name\": \"blink\", \"issue\": 12853011, \"mastername\": \"chromium.testing.master\", \"patchset\": 1, \"project\": \"chrome\", \"recipe\": \"blink_trybot\", \"repository\": \"\", \"requester\": \"commit-bot@chromium.org\", \"revision\": \"HEAD\", \"rietveld\": \"https://codereview.chromium.org\", \"root\": \"src/third_party/WebKit\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}",
       "--run-python-script",
       "[BUILD]/scripts/slave/chromium/layout_test_wrapper.py",
       "--target",
       "Release",
       "-o",
       "[SLAVE_BUILD]/layout-test-results",
@@ skipping 70 matching lines @@
       "-u",
       "print \"ALL IS WELL\""
     ],
     "name": "webkit_tests",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@python.inline@print \"ALL IS WELL\"@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   }
 ]