Fix #1388, mark the add-on as multiprocess compatible
Fix #1388, mark the add-on as multiprocess compatible

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