fix: simpleFullscreen window should be on top of other OS X menu bars. (#14881)

If an app has no menu bar (because `app.dock.hide()` has been called),
OS X will still render the menu bar of the previously-focused app.

This commit ensures simpleFullscreen windows will be drawn on top of
that menu bar by setting their level to NSPopUpMenuWindowLevel while
simpleFullscreen mode is active.

Ref: https://github.com/electron/electron/issues/11468

atom/browser/native_window_mac.h

@@ -209,6 +209,7 @@ class NativeWindowMac : public NativeWindow {
   bool was_maximizable_ = false;
   bool was_movable_ = false;
   NSRect original_frame_;
+  NSInteger original_level_;
   NSUInteger simple_fullscreen_mask_;
 
   base::scoped_nsobject<NSColor> background_color_before_vibrancy_;