ENYO-451: Add "webkitvisibilitychange" event to enyo.dispatcher.events

Enyo-DCO-1.1-Signed-off-by: Mikyung Kim <mikyung27.kim@lge.com>
ENYO-451: Added visibilitychange to support other browsers

Enyo-DCO-1.1-Signed-off-by: Mikyung Kim <mikyung27.kim@lge.com>