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Add the infrastructure to remove expired data by rewriting LevelDB
instances.
The integration with actual storage implementation will be added in
followup Cls.

Bug: 823071
Change-Id: Iff1b93481468423d292266f16a45f6ef4ac6e7f2
Reviewed-on: https://chromium-review.googlesource.com/1152917
Commit-Queue: Christian Dullweber <dullweber@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587980}