Revert 168245 "Drop background color optimization for composited..."

trunk/LayoutTests/compositing/layer-creation/spanOverlapsCanvas-expected.txt

 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (drawsContent 1)
       (children 2
         (GraphicsLayer
           (position 8.00 8.00)
           (bounds 100.00 50.00)
         )
         (GraphicsLayer
           (position 8.00 8.00)
           (bounds 100.00 50.00)
           (contentsOpaque 1)
-          (drawsContent 1)
-          (backgroundColor #008000)
         )
       )
     )
   )
 )