Issue repaints for RenderLayers that get new composited backings after

LayoutTests/compositing/repaint/fixed-pos-inside-composited-intermediate-layer-expected.txt

 (GraphicsLayer
   (bounds 785.00 30020.00)
   (children 1
     (GraphicsLayer
       (bounds 785.00 30020.00)
       (contentsOpaque 1)
       (drawsContent 1)
       (children 1
         (GraphicsLayer
           (children 2
             (GraphicsLayer
               (position 8.00 8.00)
               (bounds 500.00 500.00)
               (contentsOpaque 1)
               (drawsContent 1)
               (backgroundColor #00FFFF)
             )
             (GraphicsLayer
               (position 20.00 20.00)
               (bounds 100.00 30000.00)
               (drawsContent 1)
               (repaint rects
-                (rect 45.00 45.00 50.00 150.00)
+                (rect 25.00 25.00 50.00 150.00)
               )
             )
           )
         )
       )
     )
   )
 )