Update JS code for Switch Access Menu Panel to use ES2016.

This is a pure refactor, with no change to code behavior.

Bug: None
Change-Id: Ib864a858b2de4aca6619ad8b002d59d075f94fa1
Reviewed-on: https://chromium-review.googlesource.com/c/1428602
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Auto-Submit: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625322}
2 files changed