Remove (Native)Widget::StackBelow().

ui/views/widget/widget.cc

Index: ui/views/widget/widget.cc
diff --git a/ui/views/widget/widget.cc b/ui/views/widget/widget.cc
index 93eafed55398cc5eabc99f394ae65702bbc08423..68e6acd9c8ad63b2af337303d34de31e8f8fb041 100644
--- a/ui/views/widget/widget.cc
+++ b/ui/views/widget/widget.cc
@@ -553,10 +553,6 @@ void Widget::StackAtTop() {
   native_widget_->StackAtTop();
 }
 
-void Widget::StackBelow(gfx::NativeView native_view) {
-  native_widget_->StackBelow(native_view);
-}
-
 void Widget::SetShape(std::unique_ptr<SkRegion> shape) {
   native_widget_->SetShape(std::move(shape));
 }