Added a read pixels test. Change-Id: Ia4c5a0290e62e2c312d86fa18ef4e76d782a4a41 Reviewed-on: https://chromium-review.googlesource.com/179357 Reviewed-by: Nicolas Capens <nicolascapens@chromium.org> Commit-Queue: Nicolas Capens <nicolascapens@chromium.org> Tested-by: Nicolas Capens <nicolascapens@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>