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This is just in preparation for future changes to how this class uses
IndexedDB.

Change-Id: Ic10969e970f90a62db064c2fa53a388ff63658e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2020564
Auto-Submit: enne <enne@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Commit-Queue: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735134}