fix: don't do self-destroy in LibnotifyNotification::Dismiss() (#41691)

Callers of Notification::Dismiss() assume that the notification
instance is not deleted after the call, but this was not the case
for LibnotifyNotification:
- Destroy() would get `this` deleted.
- notify_notification_close() in portal environment triggers
LibnotifyNotification::OnNotificationClosed(), and finally calls
Destroy()

This patch removes all Destroy() in Dismiss(), and adds a boolean
to tell whether notify_notification_close() is running, to avoid crash
under portal environment.

Fixes #40461.

shell/browser/notifications/linux/libnotify_notification.h

@@ -33,6 +33,7 @@ class LibnotifyNotification : public Notification {
   RAW_PTR_EXCLUSION NotifyNotification* notification_ = nullptr;
 
   ScopedGSignal signal_;
+  bool on_dismissing_ = false;
 };
 
 }  // namespace electron