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Note that the main logic to handle focus transitions from status area
--> shelf was added in CL 1475149. This makes some small tweaks and
mainly adds the logic for transitions in the other direction.

This is a little more involved because we need to handle situations
where the overflow bubble is showing.

This also adds some extensive tests covering focus-cycling between all
these elements.

You can see a screencast of what this looks like in
    https://bugs.chromium.org/p/chromium/issues/detail?id=753409#c34

Bug: 753409
Change-Id: Ia903c27f54226b4d67cf796d2d076f9da72225a9
Reviewed-on: https://chromium-review.googlesource.com/c/1477464
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634723}