[GTK] Load the llvmpipe (Mesa) libraries when running the tests with Xvfb.
https://bugs.webkit.org/show_bug.cgi?id=134070

Reviewed by Martin Robinson.

* Scripts/webkitpy/port/base.py:
(Port.__init__): Set default _jhbuild_wrapper variable.
(Port._should_use_jhbuild): Implement generic check.
* Scripts/webkitpy/port/efl.py: Rename to jhbuild_wrapper variable
to make it consistent with the other ports.
* Scripts/webkitpy/port/gtk.py:
(GtkPort.__init__): Use the new _should_use_jhbuild() method.
(GtkPort.setup_environ_for_server): Get the path for llvmpipe_libgl_path by
querying the jhbuild with printenv. Also check that the path contains
the libraries we expect. Preload this path only for XvfbDriver.
* Scripts/webkitpy/port/xvfbdriver.py:
(XvfbDriver.check_driver): Check for the jhbuild wrapper and use it.
(XvfbDriver._start): The environment for llvmpipe_libgl_path is now
configured at setup_environ_for_server()
* Scripts/webkitpy/port/xvfbdriver_unittest.py:
(XvfbDriverTest.make_driver): Get the port default environment.
(XvfbDriverTest.test_start_no_pixel_tests): The expected log now
should include the environment that is passed when running Xvfb.
(XvfbDriverTest.test_start_pixel_tests): Ditto.
(XvfbDriverTest.test_start_arbitrary_worker_number): Ditto.
(XvfbDriverTest.test_start_next_worker): Ditto.
* gtk/jhbuild.modules: Rename checkoutdir to generic "Mesa" to
prevent future problems when the version is raised. Add also a
comment with a warning.
* gtk/jhbuildrc: The path LLVMPIPE_LIBGL_PATH should be the one
that has the llvm optimized Gallium libGL libraries.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@172830 268f45cc-cd09-0410-ab3c-d52691b4dbfc