Make DocumentTabModelImpl aware of zombie Activities

With the CipherKeyActivity sitting in between DocumentActivity and
IncognitoDocumentActivity, it's become possible to end up in situations
where an Activity doesn't have it's onStop() method called but still
be in the process of being finished().  This leads to situations
where the Incognito tab disappears from the Android Recents list but
is not yet dead.

I'm guessing there are other potential situations where this could happen,
especially since it has manifested across a wide swathe of Android L versions,
so the DocumentTabListImpl is being changed to account for zombie tabs
that should be dead but aren't yet.  These are kept in the TabList until
the DocumentTabModelSelector is informed that the Activity is really, truly
destroyed by the ApplicationStatusListener.

BUG=443330

Review URL: https://codereview.chromium.org/818483002

Cr-Commit-Position: refs/heads/master@{#309114}

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