Remove (Native)Widget::StackBelow().

ui/views/widget/native_widget_mac.mm

Index: ui/views/widget/native_widget_mac.mm
diff --git a/ui/views/widget/native_widget_mac.mm b/ui/views/widget/native_widget_mac.mm
index b1e67e8c5da32d7b029525e910ca24ec9524f40e..87e9562bbd184d6385cd670599efe59731ace937 100644
--- a/ui/views/widget/native_widget_mac.mm
+++ b/ui/views/widget/native_widget_mac.mm
@@ -332,10 +332,6 @@ void NativeWidgetMac::StackAtTop() {
   NOTIMPLEMENTED();
 }
 
-void NativeWidgetMac::StackBelow(gfx::NativeView native_view) {
-  NOTIMPLEMENTED();
-}
-
 void NativeWidgetMac::SetShape(std::unique_ptr<SkRegion> shape) {
   NOTIMPLEMENTED();
 }