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This is more prep for Mac support. The current root is a WindowElement which is
backed by aura::Window.

Bug: 769352
Change-Id: Ie7e05fb528f9f54fc07bc5ce96397715e0e34f1e
Reviewed-on: https://chromium-review.googlesource.com/876307
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537341}