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This is required because when the virtual desks takes screenshot,
old layer is already detached, so it'll be copyed as a black.

Bug: 1083951
Test: maually testsed. See bug for repro step.

Change-Id: Ia26398d2d88557a70a970748623f665ac065c46d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2501970Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821510}