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Currently if the bubble is open, you cannot focus the items in the bubble
using the keyboard. Also, if there are for example 4 items in the bubble
and the bubble is closed, you have to tab 4 times to get from the last
item on the main shelf to the first item.

This cl makes the overflow bubble activatable, and focuses it when the
index is out of visible bounds on the main shelf. The main shelf regains
focus when the index is out of visible bounds on the overflow shelf.
This cl also fixes when the overflow bubble is closed, there will not
be need to press tab extra to loop back to the beginning.

Test: add tests
Bug: 854313
Change-Id: Ib36939952488ae23300062300bdc43a7b2a1e5fc
Reviewed-on: https://chromium-review.googlesource.com/1125317Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574964}