Destroy WebContentsDelegateQt before WebContents, since it
references already partly deleted WebContentsAdapterClient object.
This prevents calls by navigation handling on already destructed
web contents adapter client.

Task-number: QTBUG-65647
Task-number: QTBUG-47945
Change-Id: I0ed5887b337a43ab89ecbfe05130691b5f1f37ec
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>