Bug 1845150 - Update notificationbox usage for process hang notificat…

…ion r=reusable-components-reviewers,tgiles

Can be tested locally using this snippet (which I think I took from a test...):

```js
let addonId = "test";
let policy = new WebExtensionPolicy({
    name: "Scapegoat",
    id: addonId,
    mozExtensionHostname: Services.uuid.generateUUID().number.slice(1, -1),
    baseURL: "file:///",
    allowedOrigins: new MatchPatternSet([]),
    localizeCallback() {},
});
policy.active = true;
ProcessHangMonitor.showNotification(window, { addonId, scriptBrowser: { browsingContext: { watchedByDevTools: false }} });
```

Differential Revision: https://phabricator.services.mozilla.com/D194316

browser/modules/ProcessHangMonitor.sys.mjs

@@ -472,7 +472,7 @@ export var ProcessHangMonitor = {
   /**
    * Show the notification for a hang.
    */
-  showNotification(win, report) {
+  async showNotification(win, report) {
     let bundle = win.gNavigatorBundle;
 
     let buttons = [
@@ -563,8 +563,10 @@ export var ProcessHangMonitor = {
       });
     }
 
-    win.gNotificationBox
-      .appendNotification(
+    // Sometimes the window may have closed already, in which case we won't
+    // be able to create a message bar so we need to handle any related errors.
+    try {
+      let hangNotification = await win.gNotificationBox.appendNotification(
         "process-hang",
         {
           label: message,
@@ -577,8 +579,11 @@ export var ProcessHangMonitor = {
           },
         },
         buttons
-      )
-      .setAttribute("notification-tag", notificationTag);
+      );
+      hangNotification.setAttribute("notification-tag", notificationTag);
+    } catch (err) {
+      console.warn(err);
+    }
   },
 
   /**

browser/modules/test/browser/browser_ProcessHangNotifications.js

@@ -183,7 +183,7 @@ add_task(async function waitForScriptTest() {
   });
 
   // Click the "Close" button this time, we shouldn't get a callback at all.
-  notification.currentNotification.closeButton.click();
+  notification.currentNotification.closeButtonEl.click();
 
   // send another hang pulse, we should not get a notification here
   Services.obs.notifyObservers(hangReport, "process-hang-report");