[color] Avoid using the GTK NativeTheme for Incognito browsers

In crrev.com/c/4542720 we switched from using the dark NativeTheme
instance to falling back to the Widget's ColorMode.

However linux-specific GTK / QT native themes are still forcefully
set if present. This causes light-mode linux themes to render the
incognito browser in a light theme.

This CL forces the native ui instance if running in incognito mode.
With this change incognito mode will fall back to respecting the
Widget's ColorMode, regardless of system theme.

Bug: 1448785
Change-Id: I9c4ebbf8cf24ddd360dc70ac863c91a514b7a1c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4567568


Reviewed-by: Thomas Lukaszewicz <tluk@chromium.org>
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1149404}