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This change is in preparation for creating the Switch Access context
menu, which will behave similarly to the ChromeVox Panel.

This is a pure refactor with no functional changes.

Bug: 864796
Change-Id: I53c34722b97acca482514132c84894c89171c2a2
Reviewed-on: https://chromium-review.googlesource.com/1144308
Commit-Queue: Zach Helfinstein <zhelfins@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577278}