Use OSMesa when running Sky tests

This CL switches us to use OSMesa rather than the system GL implementation when
running the Sky tests. This makes the tests more machine-independent and also
makes them work properly in Chromoting and in VirtualBox.

R=ojan@chromium.org, esprehn@chromium.org

Review URL: https://codereview.chromium.org/680243002

tools/webkitpy/layout_tests/port/base.py

@@ -227,7 +227,7 @@ class Port(object):
             return ['--dump-render-tree']
         if driver_name == self.MOJO_SHELL_NAME:
             return [
-                '--args-for=mojo://native_viewport_service/ --use-headless-config',
+                '--args-for=mojo://native_viewport_service/ --use-headless-config --use-osmesa',
                 '--content-handlers=text/sky,mojo://sky_viewer/',
                 '--url-mappings=mojo:window_manager=mojo:sky_tester',
                 'mojo:window_manager',