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Some of this cleanup will facilitate replacing ButtonListener with
callbacks in a future CL.

Bug: none
Change-Id: Ieefc3be0238bdaa95fc171b7ee7c55e4f8e41a90
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2472899
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Richard Knoll <knollr@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818987}