[GTK] Support dark mode and high contrast settings

This makes the omnibox dark in dark themes.

Summary of changes:
* Call set_dark_mode() in NativeThemeGtk.
* Call set_high_contrast() in NativeThemeGtk.
* Remove OmniboxTint::NATIVE.  This was only used on the GTK theme, but we want
  the setting to be based on native_theme->SystemDarkModeEnabled() now.  In the
  future, the omnibox will by themable using the GTK theme via ThemeService.

BUG=863230
R=pkasting

Change-Id: Ia5c9116c955fcba09eed63e0e11e31e954972e48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1629027
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663325}
4 files changed