Fix menu corners: the menu background was being painted as a rect when in fact,

ui/native_theme/native_theme.h

Index: ui/native_theme/native_theme.h
diff --git a/ui/native_theme/native_theme.h b/ui/native_theme/native_theme.h
index 0c5499ec4e12e6ed2c8b521da3814fdfb2d4f571..2958609dda7f76cb815080765a378c833f925b27 100644
--- a/ui/native_theme/native_theme.h
+++ b/ui/native_theme/native_theme.h
@@ -135,6 +135,10 @@ class NATIVE_THEME_EXPORT NativeTheme {
     bool has_gutter;
   };
 
+  struct MenuBackgroundExtraParams {
+    int corner_radius;
+  };
+
   struct ProgressBarExtraParams {
     double animated_seconds;
     bool determinate;
@@ -190,6 +194,7 @@ class NATIVE_THEME_EXPORT NativeTheme {
     MenuItemExtraParams menu_item;
     MenuListExtraParams menu_list;
     MenuSeparatorExtraParams menu_separator;
+    MenuBackgroundExtraParams menu_background;
     ProgressBarExtraParams progress_bar;
     ScrollbarArrowExtraParams scrollbar_arrow;
     ScrollbarTrackExtraParams scrollbar_track;