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Since we have crostini_pref_names, the kCrostiniRegistryPref for the
Crostini registry should be in there instead of being separated.

Change-Id: Ia89192dc2ac11c9cd38bf79725fc3951e3d99211
Reviewed-on: https://chromium-review.googlesource.com/1179510Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Commit-Queue: Timothy Loh <timloh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584638}