chore: add SetGTKDarkThemeEnabled(enabled) internal helper to allow dynamic theme selection on linux (#19964)

This is just a after-creation setter for the `darkTheme` constructor option.  This is delibrately
a method and not a property as there is no getter.

shell/browser/native_window.h

@@ -228,6 +228,8 @@ class NativeWindow : public base::SupportsUserData,
                            const std::string& display_name);
   virtual void CloseFilePreview();
 
+  virtual void SetGTKDarkThemeEnabled(bool use_dark_theme) = 0;
+
   // Converts between content bounds and window bounds.
   virtual gfx::Rect ContentBoundsToWindowBounds(
       const gfx::Rect& bounds) const = 0;