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The OfflinePageModel likes to call OfflinePageDeleted when entries are
refreshed or otherwise cleaned up. When this happens it comes with an
OfflinePageAdded event (before the deletion) with a different offline
page id but the same url. Thus, the offline helper entry is removed
whenever that happens because we are not keying on offline page id.

This CL makes it so we may trigger a preview attempt on a truly
deleted page, but only for a single session. This comes at the benefit
of triggering on all true positives.

Bug: 969163
Change-Id: I4cf9d303ae997f1acba46652a48112bf6d3afabd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715529Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680163}