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This is a servce-side class that can create shared images in the form of
mailboxes. This is the service-side implementation of the upcoming
SharedImageInterface, which is a stateless and thread-safe API to create
such shared images from the client side (introduced in a follow-up).

Bug: 870116

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: Id0db4300a57e02ccbd46b5b328b534d8cd3f2dc8
Reviewed-on: https://chromium-review.googlesource.com/1159384Reviewed-by: Victor Miura <vmiura@chromium.org>
Commit-Queue: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580065}