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This was not being saved, resulting in new child RWHViewMacs having
their parent ui::Layer set to nullptr.

Bug: 859834
Change-Id: I8e66370381406d8cc1dfb2aac9d81397c1775831
Reviewed-on: https://chromium-review.googlesource.com/1148940Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577958}