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The goal is to make it easy to verify that an aura::Window always
has a layer once initialized.

Bug: 738387
Change-Id: I09c4972006c8a4b86fcde0e39fc12291187e0a78
Reviewed-on: https://chromium-review.googlesource.com/774909
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517418}