Make the control buttons on a notification always show on non Chrome OS platform.

ui/arc/notification/arc_custom_notification_view.cc

Index: ui/arc/notification/arc_custom_notification_view.cc
diff --git a/ui/arc/notification/arc_custom_notification_view.cc b/ui/arc/notification/arc_custom_notification_view.cc
index 2ea6ffef8f6abb476e9845e13c4bb5a582968c62..03538dd5ac0be8ee224294db42ed87d3c2af5be1 100644
--- a/ui/arc/notification/arc_custom_notification_view.cc
+++ b/ui/arc/notification/arc_custom_notification_view.cc
@@ -241,6 +241,7 @@ ArcCustomNotificationView::ArcCustomNotificationView(ArcNotificationItem* item)
       notification_key_(item->GetNotificationKey()),
       event_forwarder_(new EventForwarder(this)) {
   SetFocusBehavior(FocusBehavior::ALWAYS);
+  set_notify_enter_exit_on_child(true);
 
   item_->IncrementWindowRefCount();
   item_->AddObserver(this);