[//gpu] Add unittest of ClientSharedImage::MakeUnowned()

Bug: 40286368
Change-Id: I684949400fe82567f1e4db3dd0c2a4f883e8e844
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5424771


Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Auto-Submit: Colin Blundell <blundell@chromium.org>
Commit-Queue: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282609}