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This enforces thread-safety by design via transactions for any
implementation code accessing shared SafeBrowsingDatabase members.

Previously this was enforced by assuming everybody would make proper
usage of |lookup_lock_| but this was deemed insufficient as programming
mistakes proved easy to make twice.

One of the subtleties of this transactional model (which made the previous
ad-hoc locking even harder to prove (and keep) correct) is that only the
main database thread is allowed to modify this state, allowing for
unlocked reads on the main thread which are important to avoid
contention when flushing to disk.

BUG=440517

Review URL: https://codereview.chromium.org/794273002

Cr-Commit-Position: refs/heads/master@{#309611}