[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..f9076462b4302949b97e1b47b1dbb2846bd6dc1e 100644
--- a/ui/views/widget/widget.h
+++ b/ui/views/widget/widget.h
@@ -561,7 +561,7 @@ class VIEWS_EXPORT Widget : public internal::NativeWidgetDelegate,
     return const_cast<ui::NativeTheme*>(
         const_cast<const Widget*>(this)->GetNativeTheme());
   }
-  const ui::NativeTheme* GetNativeTheme() const;
+  virtual const ui::NativeTheme* GetNativeTheme() const;
 
   // Returns the FocusManager for this widget.
   // Note that all widgets in a widget hierarchy share the same focus manager.