Suppose the user overrides QWebEngineView::dragEnterEvent without
calling the base implementation and without overriding dragLeaveEvent.
Then our implementation will notify chromium about the drag leave
without having ever seen a drag entering and crash. Only notify chromium
about leave/drop/move events if we've notified it about the drag enter
before. Also, catch the case where the user overrides dragLeaveEvent
without calling the base implementation.

Task-number: QTBUG-54896
Change-Id: Ib958040e5fa7ecab86bac9b724d478c81a521fcc
Reviewed-by: Michael Brüning <michael.bruning@qt.io>