Update geofencing layout tests to use mock service.

LayoutTests/http/tests/geofencing/resources/worker-passes-events-back.js

+var port;
+
+self.addEventListener('message', function(event) {
+  port = event.data.port;
+  port.postMessage('setup');
+});
+
+self.addEventListener('geofenceenter', function(event) {
+  port.postMessage({event: 'geofenceenter', id: event.region.id});
+});
+
+self.addEventListener('geofenceleave', function(event) {
+  port.postMessage({event: 'geofenceleave', id: event.region.id});
+});