Destruct BrowserContextAdapter together with WebEngineContext in widget

BrowserContext needs IO thread to be destructed properly. Without
WebEngineContext dependency it would be destructed together with the
corresponding QWebEngineProfile which can outlive the WebEngineContext.

Task-number: QTBUG-62147
Change-Id: Ia34215f379c6c803499848bf009e25a64b5cdb98
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>