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

trunk/LayoutTests/compositing/layer-creation/overflow-scroll-overlap-expected.txt

 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (drawsContent 1)
       (children 3
         (GraphicsLayer
           (position 8.00 8.00)
@@ skipping 12 matching lines @@
           )
         )
         (GraphicsLayer
           (position 23.00 23.00)
           (bounds 285.00 200.00)
           (children 1
             (GraphicsLayer
               (position 20.00 45.00)
               (bounds 210.00 100.00)
               (contentsOpaque 1)
-              (drawsContent 1)
-              (backgroundColor #0000FF)
             )
           )
         )
       )
     )
   )
 )