This cl's switches cc_unittests and viz_unittests from OSMesa
to SwiftShader and fixes other issues related to that change.

Bug chromium:848035

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ib1b58e02d4df4a687ef7be57284c06097bdeca94
Reviewed-on: https://chromium-review.googlesource.com/1070490Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Eric Karl <ericrk@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572442}