Notice that one might consider removing the IGUIObject reference from CGUIText, but that this would be more involved cleanup, whereas this diff is a preparation for D2325 (amongst D2332).
See that the code is equivalent and measurably less complex (less nr of arguments) and less prone to passing the wrong object.
Notice that the other arguments like Font and BufferZone are not defined in the IGUITextOwner but the derived class such as CText, hence this is the only argument that can be removed this way here.