Disable solid background color optimization for composited layers.

LayoutTests/compositing/geometry/limit-layer-bounds-fixed-positioned-expected.txt

 Text here
 (GraphicsLayer
   (bounds 785.00 1021.00)
   (children 1
     (GraphicsLayer
       (bounds 785.00 1021.00)
       (contentsOpaque 1)
       (children 3
         (GraphicsLayer
           (position 29.00 134.00)
           (bounds 100.00 100.00)
         )
         (GraphicsLayer
           (position 29.00 134.00)
           (bounds 100.00 100.00)
           (contentsOpaque 1)
+          (drawsContent 1)
+          (backgroundColor #FF0000)
         )
         (GraphicsLayer
           (position 0.00 113.00)
           (bounds 150.00 142.00)
           (drawsContent 1)
         )
       )
     )
   )
 )