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

ui/views/views_delegate.cc

Index: ui/views/views_delegate.cc
diff --git a/ui/views/views_delegate.cc b/ui/views/views_delegate.cc
index a6ce9cc5c27571fc718c24d132ab9f198daa8e01..5edc5fd98ae9bdf9929c31b9d82c56cd1e39ed79 100644
--- a/ui/views/views_delegate.cc
+++ b/ui/views/views_delegate.cc
@@ -120,6 +120,10 @@ scoped_refptr<base::TaskRunner> ViewsDelegate::GetBlockingPoolTaskRunner() {
   return nullptr;
 }
 
+ui::NativeTheme* ViewsDelegate::GetNativeThemeOverride(const Widget* widget) {
+  return nullptr;
+}
+
 ViewsDelegate::ViewsDelegate()
     : views_tsc_factory_(new ViewsTouchEditingControllerFactory) {
   DCHECK(!views_delegate);