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Allows user drop down menu to activatble so that it can be cycled
through.
Add a custom focus search so the drop down menu closes after all items
have been cycled through.
Migrate Closure to OnceClosure.
Make drop down menu a transient child of system bubble.

Test: manual
Bug: 799401
Change-Id: I67af2bb3e58c88f6926ab65ecf565d69325d00b8
Reviewed-on: https://chromium-review.googlesource.com/1038656Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555756}