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Add a new test target gl_unittests_ozonex that runs with
--ozone-platform=x11. The Ozone CrOS bot will now run gl_unittests with
both Ozone headless and Ozone X11.

The test requires an X server to be running on the bot. The mb.py script
is modified to look for GN arg use_ozone=true and runtime flag
--ozone-platform=x11. In this case, it runs the test using xvfb.

Original commit crrev.com/2493063003 was reverted due to
crbug.com/665895. There was no dependency on xdisplaycheck by the new
test target that needs it. This caused problems during the isolate step
when xdisplaycheck wasn't found.

BUG=664570

Review-Url: https://codereview.chromium.org/2509033006
Cr-Commit-Position: refs/heads/master@{#435000}