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In bringing Switch Access up to the visual specification, the menu is
being rewritten directly in views/.

This change adds the remaining buttons that may appear in the menu, as
well as adding the ability to specify which buttons are displayed and
firing an event when the button is pressed.

This code is not yet used by the feature; it will be fully incorporated
in a follow-up change.

Relnotes: N/A
Bug: 973719
Change-Id: Ibc3c820b2ad791ee8107ce86da75ba52471e71ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2151826
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Auto-Submit: Anastasia Helfinstein <anastasi@google.com>
Cr-Commit-Position: refs/heads/master@{#762077}