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

trunk/LayoutTests/compositing/geometry/flipped-writing-mode-expected.txt

 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (drawsContent 1)
       (children 1
         (GraphicsLayer
           (position 18.00 10.00)
           (bounds 250.00 200.00)
           (contentsOpaque 1)
-          (drawsContent 1)
-          (backgroundColor #C0C0C0)
           (children 1
             (GraphicsLayer
               (position 35.00 10.00)
               (anchor 0.74 0.50)
               (bounds 195.00 100.00)
               (drawsContent 1)
               (backgroundColor #0000FF)
             )
           )
         )
       )
     )
   )
 )