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The default storage for OverlayUserPrefStore which is used for
incognito preferences is the persistent storage in the regular profile,
and preference types that should not be stored there must be enumerated
in CreateIncognitoPrefServiceSyncable.

This is changed so that the default storage is in memory, and
preferences that need a persistent storage would need enumeration.

Bug: 861722
Change-Id: Ia4c8f655752a07f81fb6d35685876cd252118999
Reviewed-on: https://chromium-review.googlesource.com/1128753
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575613}