https://bugs.webkit.org/show_bug.cgi?id=122089

Reviewed by Gyuyoung Kim.

Make the Xvfb screen depth overridable by checking if the XVFB_SCREEN_DEPTH environment variable
was set. The default value that's used if the variable wasn't set is 24.

* Scripts/webkitpy/port/xvfbdriver.py:
(XvfbDriver._xvfb_screen_depth):
(XvfbDriver._start):
* Scripts/webkitpy/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.test_start_next_worker):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@156640 268f45cc-cd09-0410-ab3c-d52691b4dbfc