Theme switching broke due to the introduction of "preferred color
scheme" into native theme. See Chromium change below.

On top of SetDarkMode that we already were doing, added
SetPreferredColorScheme that sets preferred color scheme to our
selection.

Also, had to patch ChromeContentBrowserClient to catch switching
preferred color in web prefs when the system color is switched. That
gets triggered by the system color change, for example via control panel
on Windows. The Windows specific implementation watches a registry value
and updates preferred color scheme in the native theme. Would be great
to intercept it there, but in that layer we don't have access to our
setting stored in profile prefs, from what I can tell. Hence, the patch
to ChromeContentBrowserClient where the setting is used. Ugly.
cc: @simonhong perhaps you have better ideas?

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