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This allows us prevent updating the visibility of the mirrored backdrop
in the desk preview view based on changes in the source backdrop's layer
visibility.

BUG=978917

Change-Id: I98c80a7c697f61fff37a06aab377f157099ded17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1693186Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675862}