[MD] Implement incognito colors as a NativeTheme (for Aura).

ui/views/widget/widget.h

Index: ui/views/widget/widget.h
diff --git a/ui/views/widget/widget.h b/ui/views/widget/widget.h
index 301c0aff1a1cfac6dd64bdda54583b6534109feb..ac9d05c707a8465bb544cc9680a8e9732023e554 100644
--- a/ui/views/widget/widget.h
+++ b/ui/views/widget/widget.h
@@ -240,6 +240,8 @@ class VIEWS_EXPORT Widget : public internal::NativeWidgetDelegate,
     // When set, this value is used as the Widget's NativeWidget implementation.
     // The Widget will not construct a default one. Default is NULL.
     NativeWidget* native_widget;
+    // If provided, sets the native theme for this widget.
+    ui::NativeTheme* native_theme;
     // Aura-only. Provides a DesktopWindowTreeHost implementation to use instead
     // of the default one.
     // TODO(beng): Figure out if there's a better way to expose this, e.g. get
@@ -859,6 +861,10 @@ class VIEWS_EXPORT Widget : public internal::NativeWidgetDelegate,
 
   internal::NativeWidgetPrivate* native_widget_;
 
+  // If non-null, the native theme for this widget. Otherwise the native theme
+  // comes from |native_widget_|.
+  ui::NativeTheme* native_theme_;
+
   base::ObserverList<WidgetObserver> observers_;
 
   base::ObserverList<WidgetRemovalsObserver> removals_observers_;