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This is a reland of a61d2bb4

The flaky test has been excluded from this patch, will be
reintroduced in a separate patch.

Original change's description:
> Reland "Avoid recycling sync tabs if commit pending"
>
> This is a reland of 62414cfc
>
> Test improved to avoid flakes.
>
> Original change's description:
> > Avoid recycling sync tabs if commit pending
> >
> > 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}
>
> TBR=treib@chromium.org
>
> Bug: 882489
> Change-Id: Ib3f3ff9e4d620512435d9b22ba7b2c338f92203f
> Reviewed-on: https://chromium-review.googlesource.com/c/1357086
> Reviewed-by: Mikel Astiz <mastiz@chromium.org>
> Commit-Queue: Mikel Astiz <mastiz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#612980}

TBR=treib@chromium.org

Bug: 882489
Change-Id: Iaa6490f896257baa8a843ca0e7c92f2d7dc11c8c
Reviewed-on: https://chromium-review.googlesource.com/c/1356808Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613028}