pyauto_functional_tests on linux tryservers should run under xvfb
pyauto_functional_tests on linux tryservers should run under xvfb

pyauto_functional_tests for linux is configured by default to not run under xvfb
(because QA bots using Google Chrome builds want to run tests under real X).
But for try jobs, run it under xvfb. This is how master.chromium.pyauto
is also configured to do. |use_xvfb_on_linux| is unused for other tests
and platforms.


BUG=None
TEST=None
Review URL: https://chromiumcodereview.appspot.com/10408008

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@137744 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build