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Add a custom GLSurfaceOSMesaPng that outputs to file. Unfortunately the
GPU sandbox will block this from writing to file, so it only works
without the GPU sandbox.

Bug: 783792
Change-Id: I868702aa5dab81bffb8ea4867d659225255e5d50
Reviewed-on: https://chromium-review.googlesource.com/758608
Commit-Queue: kylechar <kylechar@chromium.org>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516061}