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When running with mus, and mus is not hosting viz, aura has to generate
the LocalSurfaceId. It needs to make sure the root window always has a
valid LocalSurfaceId, because that's what is fed into ui::Compositor,
and without a valid id, it never shows anything on screen.

BUG=786453

Change-Id: Ie075246937f7e607afdc945ea1cea0b021595e50
Reviewed-on: https://chromium-review.googlesource.com/807886Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521762}