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The Dark/Light mode toggle in System Settings appears to be intended
for UWP apps, so this change needs to be slightly more creative about
checking this state.

This change checks and monitors AppsUseLightTheme in HKCU's theme area,
which is not an API guarantee. Future versions of Windows could change
the way this is persisted and subsequently break this change.

BUG=838670

Change-Id: Ic84341ae69e2bb9cd988f9bea027052033c66360
Reviewed-on: https://chromium-review.googlesource.com/c/1429963Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625331}