Add the future storage framework for the BudgetDatabase. This isn't
currently plugged into the BackgroundBudgetService, but will be in future
work.

It will be able to store objects that fit the Budget protobuf type, which
has pieces of budget that have been allocated, an expiration time for those
pieces, and how much (if any) in debt a service worker is.

BUG=617971

Review-Url: https://codereview.chromium.org/2107173002
Cr-Commit-Position: refs/heads/master@{#405975}
9 files changed