Fix memory corruption in XHR implementation

Now that the member data of Object is also on the GC heap, it is even more
important to call the base implementation of markObjects when re-implementing
it ;-)

Fixes also three XHR auto tests (text, cdata and stateChangeCallingContext)

Change-Id: Ifd033cac64a8cf27a9186d8cd9eb94369d8a60a5
Reviewed-by: Michael Brasser <michael.brasser@live.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>