Re-enable solid background color optimization for composited layers.

LayoutTests/compositing/overflow/opt-unclipped-descendants-into-composited-scrolling-expected.txt

 This test ensures that a stacking context overflow-scroll div's unclipped descendant gets its own composited layer, which is correctly parented under the overflow-div's composited layer.
 
 (GraphicsLayer
   (bounds 1496.00 585.00)
   (children 1
     (GraphicsLayer
       (bounds 1496.00 585.00)
       (contentsOpaque 1)
       (drawsContent 1)
       (children 1
         (GraphicsLayer
           (position 28.00 20.00)
           (bounds 202.00 202.00)
           (drawsContent 1)
           (children 4
             (GraphicsLayer
               (position 1.00 1.00)
               (bounds 185.00 185.00)
               (children 1
                 (GraphicsLayer
                   (bounds 190.00 185.00)
                   (drawsContent 1)
                   (children 1
                     (GraphicsLayer
                       (position 10.00 139.00)
                       (bounds 180.00 90.00)
                       (contentsOpaque 1)
-                      (drawsContent 1)
-                      (backgroundColor #808080)
                     )
                   )
                 )
               )
             )
             (GraphicsLayer
               (position 1.00 186.00)
               (bounds 185.00 15.00)
               (drawsContent 1)
             )
             (GraphicsLayer
               (position 186.00 1.00)
               (bounds 15.00 185.00)
               (drawsContent 1)
             )
             (GraphicsLayer
               (position 186.00 186.00)
               (bounds 15.00 15.00)
               (drawsContent 1)
             )
           )
         )
       )
     )
   )
 )