Inline implementation of SysColorChangeListener into the lone user.

This can be further improved -- NativeThemeWin can treat the color and
high contrast changes separately -- but that will come later.

Bug: 431953872
Change-Id: I5cd7294f0d49d229e954e877ff3cbcb937e9c161
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6905083
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Tom Lukaszewicz <tluk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1509318}