Update the button visibility when mouse enters or exits from notification surface window

ui/arc/notification/arc_notification_content_view.cc

Index: ui/arc/notification/arc_notification_content_view.cc
diff --git a/ui/arc/notification/arc_notification_content_view.cc b/ui/arc/notification/arc_notification_content_view.cc
index f7b23554c056e1e1c37848cefa783dbf0807ecf5..6ea6011d186870031f12aa1c3be791770fb74c76 100644
--- a/ui/arc/notification/arc_notification_content_view.cc
+++ b/ui/arc/notification/arc_notification_content_view.cc
@@ -84,6 +84,12 @@ class ArcNotificationContentView::EventForwarder : public ui::EventHandler {
       ui::LocatedEvent* located_event = event->AsLocatedEvent();
       located_event->target()->ConvertEventToTarget(widget->GetNativeWindow(),
                                                     located_event);
+      if (located_event->type() == ui::ET_MOUSE_ENTERED ||
+          located_event->type() == ui::ET_MOUSE_EXITED) {
+        owner_->UpdateControlButtonsVisibility();
+        return;
+      }
+
       if (located_event->type() == ui::ET_MOUSE_MOVED ||
           located_event->IsMouseWheelEvent()) {
         widget->OnMouseEvent(located_event->AsMouseEvent());