Use OSMesa when running Sky tests
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

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