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Add a feature to enable a timestamp on the Previews infobar. This
timestamp should be shown when the previews_freshness is greater
than the finch controlled "min_staleness_in_minutes" and less than
"max_staleness_in_minutes".

BUG=718643

Review-Url: https://codereview.chromium.org/2940083003
Cr-Commit-Position: refs/heads/master@{#481018}