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

trunk/LayoutTests/compositing/squashing/squash-three-layers-expected.txt

 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (drawsContent 1)
       (children 1
         (GraphicsLayer
           (children 2
             (GraphicsLayer
               (position 100.00 100.00)
               (bounds 100.00 100.00)
               (contentsOpaque 1)
-              (drawsContent 1)
-              (backgroundColor #0000FF)
             )
             (GraphicsLayer
               (position 40.00 40.00)
               (bounds 180.00 190.00)
               (drawsContent 1)
             )
           )
         )
       )
     )
   )
 )