Disable solid background color optimization for composited layers.

LayoutTests/compositing/layer-creation/stacking-context-overlap-expected.txt

 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (children 2
         (GraphicsLayer
           (position 8.00 8.00)
           (bounds 20.00 20.00)
           (contentsOpaque 1)
+          (drawsContent 1)
+          (backgroundColor #0000FF)
         )
         (GraphicsLayer
           (position 8.00 18.00)
           (bounds 142.00 142.00)
           (drawsContent 1)
         )
       )
     )
   )
 )