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The Switch Access object has always had a singleton object that lived
in the background page. Clarify where the object lives by accessing it
through a static method on the class.

Bug: None
Change-Id: I5b131e9a5679d27b6ad10dffc342c39ec9f75edc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1875383Reviewed-by: Akihiro Ota <akihiroota@chromium.org>
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Cr-Commit-Position: refs/heads/master@{#709162}