Implement QWebEnginePage::viewportSize

Keep the size in the page itself, and ask the optionally attached
QWebEngineView only for the rect position.
When the view size change, call setViewportSize on the page like
QtWebKit does.

Change-Id: I735f1509cab9ef9b5d32bf31249a59c919939804
Reviewed-by: Andras Becsi <andras.becsi@digia.com>