=== Handle an activity request ===
View a png image:
navigator.setMessageHandlermozSetMessageHandler('activity', function(a) {
// We can't be handling an activity that isn't what we asked, because of |filters|.
var i = getImageObject();
Pick an image:
navigator.setMessageHandlermozSetMessageHandler('activity', function(a) {
var image = getImageToReturn();
if (!image) {