Migrate browsertest to use ServiceWorkerRegistration.unregister()

content/test/data/service_worker/imports_bust_memcache.html

Index: content/test/data/service_worker/imports_bust_memcache.html
diff --git a/content/test/data/service_worker/imports_bust_memcache.html b/content/test/data/service_worker/imports_bust_memcache.html
index cc0db4f494046130898a3c2aac557ca2494fb94f..2fc8af574e3144f735b42bbc4842a8f4c106792c 100644
--- a/content/test/data/service_worker/imports_bust_memcache.html
+++ b/content/test/data/service_worker/imports_bust_memcache.html
@@ -8,14 +8,18 @@
 var scope = 'imports_bust_memcache_scope/';
 var script = 'worker_with_one_import.js';
 
-navigator.serviceWorker.unregister(scope)
+navigator.serviceWorker.getRegistration(scope)
+.then(function(registration) {
+  if (registration)
+    return registration.unregister();
+})
 .then(function(_) {
   console.log('initial unregistration done');
   return navigator.serviceWorker.register(script, {scope: scope});
 })
-.then(function(_) {
+.then(function(registration) {
    console.log('initial registration done');
-   return navigator.serviceWorker.unregister(scope);
+   return registration.unregister();
 })
 .then(function(_) {
   console.log('unregistration done');