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This changes the behavior of TabNodePool::AssociateWithFreeTabNode() to
always return the smallest available ID, resulting in a more compact
distribution of sync IDs too, hence a lower number of sync entities.

This will allow, in future patches, the deletion of arbitrary tab
entities, without incurring an increase of the number of session sync
entities.

Bug: 882489
Change-Id: I11ac5167f6971dbd8239b530a87a49eb51f20261
Reviewed-on: https://chromium-review.googlesource.com/1242883Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594707}