This commit is a squash of 4 commits from #3281.

- Fix setting preferred color scheme logic for brave theme change
- Fix system dark mode change affect brave theme on Win10
    When brave theme is set to dark or light, system dark mode change
    should not affect brave theme.
- CleanUp: Delete SystemThemeSupportDarkMode
    Upstream added ui::NativeTheme::SystemDarkModeSupported().
    Replace ours with upstream one.
- Add dark mode change test by modifying reg value for Windows
    This test can verify whether theme change notification is delivered
    or not.
    If brave theme option is not default, system dark theme change should
    not trigger theme change notificagtion delivery.

Chromium change:

https://chromium.googlesource.com/chromium/src/+/67fd4d9f35650a079f6a636b5d19de77856bd2cf

commit 67fd4d9f35650a079f6a636b5d19de77856bd2cf
Author: Alison Maher <almaher@microsoft.com>
Date:   Fri Jul 12 18:34:21 2019 +0000

    Set preferred color scheme for high contrast

    This change adds logic to update the preferred color scheme based on
    the OS high contrast theme. If high contrast is not enabled, preferred
    color scheme will continue to be based on the OS dark mode state.

    A PreferredColorScheme enum was added to NativeTheme to keep track
    of the current high contrast/dark mode preferred color scheme.

    The updated preferred color scheme is used to evaluate the
    prefers-color-scheme media query.

    Bug: 970285