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Adds the SessionStorageLevelDBWrapper class, which works with the
SessionStorageMetadata and SessionStorageDataMap classes to support
shallow cloning for SessionStorage data maps.

TBR-ing jam for content/browser/BUILD.gn

TBR: jam@chromium.org
Bug: 716490
Change-Id: I1a3ad1a79f39ef001bd4489591d6e99ab0f6347f
Reviewed-on: https://chromium-review.googlesource.com/954410
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553288}