Don't always repaint on zIndex change

LayoutTests/compositing/repaint/should-not-repaint-composited-descendants-expected.txt

 position change with composited child
 
 position change with non-composited child
 
 z-index change with composited child
 
 z-index change with non-composited child
 
 opacity change with composited child
 
@@ skipping 156 matching lines @@
                   (backgroundColor #FFFF00)
                 )
               )
             )
           )
         )
         (GraphicsLayer
           (position 8.00 620.00)
           (bounds 100.00 100.00)
           (drawsContent 1)
-          (repaint rects
-            (rect 0.00 0.00 100.00 100.00)
-            (rect 0.00 0.00 100.00 100.00)
-            (rect 0.00 0.00 100.00 100.00)
-          )
         )
       )
     )
   )
 )