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Per comments on the CL to change the ownership of WebStateLists
(crrev.com/c/1796358), there are a couple of nuances to behavior
in TabModel that aren't currently covered by unit tests.

First, multiple calls to -browserStateDestroyed must be safe. Since
possible failure modes are DCHECKS or outright segfaults, the test just
creates and populates a tab model, then calls -browserStateDestroyed
twice.

Second, empty sessions should be saved (meaning that when the user has
no open tabs, restoring the session should not show any of the previously
open tabs). The test here is also straightforward: force-save a non-empty
session, close all of the tabs, force-save again, restore, and verify
that there are no open tabs.

Change-Id: I3b40d8fc6f60ca51bfaebeb704324a04d60bd7d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1819239Reviewed-by: Mohammad Refaat <mrefaat@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699238}