Expose navigator.webkitPersistentStorage.requestQuota() calls to
the API layer as a permission API. It allows the users to accept
these requests and specify a quota for persistent storage.
https://developer.chrome.com/apps/offline_storage#managing_quota



[ChangeLog] navigator.webkitPersistentStorage.requestQuota() calls
were rejected by default and now they will emit signal
quotaPermissionRequested in both WebEngineView and QWebEnginePage.

Task-number: QTBUG-56354
Change-Id: Id192577ffb403694d3051414744ded89bbfd2aa8
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>