[css-grid] Avoid double loop in positioned objects layout
Currently we were doing 2 loops to layout positioned objects:
* First we calculate the extra offsets for each object.
* Then we call layoutPositionedObjects() which does a new loop.
This patch extracts the common logic for each positioned object
in a new method layoutPositionedObject().
Now in LayoutGrid::layoutPositionedObjects() we'll call
this new method and that way we'll be doing only 1 loop
over the positioned objects.
No new tests, no change of behavior.