grabHelper: Use a round trip for focusing the default window

We may release the focus grab at any time, so it's not guaranteed
we'll be in event processing. In particular, hovering over and out
of a notification will cause this to happen, as the notification
is hidden on a timeout.

https://bugzilla.gnome.org/show_bug.cgi?id=695659

js/ui/grabHelper.js

@@ -248,7 +248,7 @@ const GrabHelper = new Lang.Class({
                 global.set_stage_input_mode(Shell.StageInputMode.FOCUSED);
         }
 
-        global.screen.focus_default_window(global.get_current_time());
+        global.screen.focus_default_window(global.display.get_current_time_roundtrip());
     },
 
     // ignoreRelease: