[TabModel closeAllTabs] calls close in a loop, which is the same call that is used to close a tab in normal operation.

Calls to close should schedule a new session to be written to disk after a delay.  Most of the time, the app shuts down before anything is written out.  But is it possible for a heavily-loaded device to take so long that the write executes?

If it's possible to lose tabs during shutdown, we should fix this.  We should consider fixing it anyways, because tearing down a TabModel shouldn't update the saved session file at all.