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Uses the new WritableUnalignedMapping class, which changes the ownership
semantics. See the bug for details on the overall plan.

This is a reland of crrev.com/c/1117696, now that mojo is initialized
for the unittests is fixed, and also removes a double-close for the
shared memory handles.

Bug: 849207
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
Change-Id: Ib065caedfed8a33fd0f112ce22a72bda4b9bf99e
Reviewed-on: https://chromium-review.googlesource.com/1158567Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Matthew Cary <mattcary@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582177}