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I was just doing a cleanup pass on VrShellDelegate, and ended up pulling
on a cleanup thread for VrSupportLevel.

I could split this into multiple changes if necessary, but that sounds
like a waste of time.

Change-Id: I2a5b962b4b7558b64f792c806314a7eb7b2e81ed
Reviewed-on: https://chromium-review.googlesource.com/1135727Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574925}