Fix #1611, deinitialize onDestroyed to undefined
Fix #1611, deinitialize onDestroyed to undefined

fzzzy/pageshot,mozilla-services/pageshot,fzzzy/pageshot,mozilla-services/pageshot,fzzzy/pageshot,mozilla-services/screenshots,mozilla-services/screenshots,mozilla-services/pageshot,mozilla-services/screenshots,mozilla-services/pageshot,mozilla-services/screenshots,fzzzy/pageshot