https://bugs.webkit.org/show_bug.cgi?id=88414

Reviewed by Dirk Pranke.

This change how the XvfbDriver choose the next display. Before
this the choosing are based on the worker number but it caused
errors when several nrwt run on the same time. This add process
based free diplay search.

* Scripts/webkitpy/layout_tests/port/xvfbdriver.py:
(XvfbDriver.__init__):
(XvfbDriver._next_free_display):
(XvfbDriver._start):
(XvfbDriver.stop):
* Scripts/webkitpy/layout_tests/port/xvfbdriver_unittest.py:
(XvfbDriverTest.make_driver):
(XvfbDriverTest.test_start_no_pixel_tests):
(XvfbDriverTest.test_start_pixel_tests):
(XvfbDriverTest.test_start_arbitrary_worker_number):
(XvfbDriverTest.test_next_free_display):
(XvfbDriverTest):
(XvfbDriverTest.test_start_next_worker):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@129891 268f45cc-cd09-0410-ab3c-d52691b4dbfc