Remove support for PushSubscription.subscriptionId.

LayoutTests/http/tests/push_messaging/resources/instrumentation-service-worker.js

 // Allows a document to exercise the Push API within a service worker by sending commands.
 
 // The port through which the document sends commands to the service worker.
 var port = null;
 
 // The most recently seen subscription.
 var lastSeenSubscription = null;
 
 self.addEventListener('message', function(workerEvent) {
     port = workerEvent.data;
@@ skipping 10 matching lines @@
                                        success: true,
                                        permission: permissionStatus });
                 }).catch(makeErrorHandler(event.data.command));
                 break;
 
             case 'subscribe':
                 self.registration.pushManager.subscribe(options).then(function(subscription) {
                     lastSeenSubscription = subscription;
                     port.postMessage({ command: event.data.command,
                                        success: true,
-                                       subscriptionId: subscription.subscriptionId,
                                        endpoint: subscription.endpoint });
                 }).catch(makeErrorHandler(event.data.command));
                 break;
 
             case 'getSubscription':
                 self.registration.pushManager.getSubscription().then(function(subscription) {
                     lastSeenSubscription = subscription;
-                    var subscriptionId = subscription ? subscription.subscriptionId : null;
+                    var endpoint = subscription ? subscription.endpoint : null;
                     port.postMessage({ command: event.data.command,
                                        success: true,
-                                       subscriptionId: subscriptionId });
+                                       endpoint: endpoint });
                 }).catch(makeErrorHandler(event.data.command));
                 break;
 
             case 'unsubscribe':
                 self.registration.pushManager.getSubscription()
                     .then(function(subscription) {
                         // We keep track of lastSeenSubscription so we can attempt to unsubscribe
                         // more than once.
                         subscription = subscription || lastSeenSubscription;
                         if (!subscription)
@@ skipping 20 matching lines @@
 });
 
 function makeErrorHandler(command) {
     return function(error) {
         var errorMessage = error ? error.message : 'unknown error';
         port.postMessage({ command: command,
                            success: false,
                            errorMessage: errorMessage });
     };
 }