When pages are deleted by delete-methods from the OfflinePageModel, registered observers are notified about that happening when OfflinePageModelTaskified::OnDeleteDone is called.

But when they are deleted by the tasks created from OfflinePageModelTaskified::RunMaintenanceTasks, no such notifications are emitted. This bug might cause OPM observers to not be informed about actual deletions and present outdated information about existing Offline Pages.

This would cause problems in two situations:
* When launching Chrome and one of these take place for the first time: all pages are queried for (i.e. when opening the Downloads Home) or an offline page is saved (i.e. last_n is triggered when the user switches tabs).
* And then for long running sessions, when either of those happen again after 30 min (the enforced delay between maintenance task runs).