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In some circumstances, |connectedScenes| can contain a scene with
no associated SceneController while cleaning up after the last OTR tab.

This CL ensures that when this happens, the nil controller isn't added
to the controller array.

(Root cause for this state is unclear; some comments are added here as
well to guide future investigations).

Bug: 1142782
Change-Id: I33617e6e2252a2124870430c2eefc2cb4158a28c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2506440
Commit-Queue: Mark Cogan <marq@chromium.org>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/master@{#822260}