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The site engagement service decays origins which have not been accessed
in a week. However, if a user isn't using Chrome at all, the decay still
occurs.

This CL addresses this issue by persisting the last recorded time that
any engagement occurred. If too long a period passes (currently 10 days)
without *any* engagement, all scores have their last accessed time
adjusted to be relative to some number of decay periods prior to now
(currently set at 1 decay period), based on their offset to the last known
engagement time.

BUG=575504

Review-Url: https://codereview.chromium.org/2082953002
Cr-Commit-Position: refs/heads/master@{#402653}