Add contentsSize API

Adds API to read contentsSize to go with scrollPosition.

This property used to be in QWebFrame in QtWebKit.

Change-Id: I498075e4c0ad916e5c96dbfb02101ce8a0c298ee
Task-number: QTBUG-48323
Reviewed-by: Michael Brüning <michael.bruning@theqtcompany.com>