Mac: Change dark mode optout logic and respond to system changes

This change hooks up the "DarkMode" feature, allowing for three states
in Mojave:
- --force-dark-mode for dark appearance unconditionally
- --enable-feature=DarkMode to track system dark mode status
- No flags/default state is light appearance unconditionally

Since we build with an SDK < 10.14, we still need the Info.plist
key, but it now must be false.

Some related changes:
- Make Omnibox tint respond to OnNativeThemeChanged
- React immediately to changes in high contrast mode setting

Bug: 850098
Change-Id: I08a2c79bb8b79c4ca17fff10bc86249e9aa484c5
Reviewed-on: https://chromium-review.googlesource.com/c/1352394
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613981}
9 files changed