Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/renderer/notification_permission_dispatcher.h

Index: content/renderer/notification_permission_dispatcher.h
diff --git a/content/renderer/notification_permission_dispatcher.h b/content/renderer/notification_permission_dispatcher.h
index 77fbb63c2a05532aeb243794109d01849ff201d2..8b1eb58adfafb09bd00e1381b9455740589c6a96 100644
--- a/content/renderer/notification_permission_dispatcher.h
+++ b/content/renderer/notification_permission_dispatcher.h
@@ -33,6 +33,9 @@ class NotificationPermissionDispatcher : public RenderFrameObserver {
                          blink::WebNotificationPermissionCallback* callback);
 
  private:
+  // RenderFrameObserver implementation.
+  void OnDestruct() override;
+
   void OnPermissionRequestComplete(
       std::unique_ptr<blink::WebNotificationPermissionCallback> callback,
       blink::mojom::PermissionStatus status);