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When a foreign session is detected to be old enough, garbage collection
should kick in and all sync entities deleted. This includes orphan tabs
as well (i.e. tabs not referenced from the header entity because the tab
was closed).

Bug: 882489
Change-Id: I9f6e0e4505cc798466294f1fa1a247818cf004b9
Reviewed-on: https://chromium-review.googlesource.com/c/1356717
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613068}