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When a tab is closed, it's possible that the corresponding history
hasn't been committed yet, and hence there is a risk that synced history
is lost if the entity is recycled (for another tab that is opened).

In this patch, and behind a feature toggle, this issue is prevented by
*not* freeing tab nodes while the sync entity is unsynced. Old tabs are
excluded from this (to avoid problems with expired history) and a max
cap is also introduced to the number of tabs in this state, in order to
avoid memory regressions.

Bug: 882489
Change-Id: I6dd796642f9553f2713a0814731897a4ffb13f0b
Reviewed-on: https://chromium-review.googlesource.com/c/1356541
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612971}