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Only display roots are considered root windows. We were doing the right
thing for remote widgets, but not for remote embeds.

This fixes Tast test ui.VirtualKeyboardOmnibox, which was failing
because the virtual keyboard uses embedding but the widget is owned
by ash.

Bug: 931574
Test: added to ash_unittests
Change-Id: I7d3bf1c5fccca596cf075969b2dfafc46426ad68
Reviewed-on: https://chromium-review.googlesource.com/c/1474261Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632717}