Re-enable solid background color optimization for composited layers.

LayoutTests/compositing/layer-creation/fixed-position-nonscrollable-body-overlap-expected.txt

 Even though we can opt-out of fixed-position compositing for unscrollable fixed-position containers, we still need to composite fixed-position layers that need compositing for other reasons such as overlap.
 
 (GraphicsLayer
   (bounds 800.00 4024.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 4024.00)
       (contentsOpaque 1)
       (drawsContent 1)
       (children 2
         (GraphicsLayer
           (position 10.00 100.00)
           (bounds 100.00 100.00)
           (contentsOpaque 1)
-          (drawsContent 1)
-          (backgroundColor #FF0000)
         )
         (GraphicsLayer
           (position 10.00 100.00)
           (bounds 100.00 100.00)
           (contentsOpaque 1)
-          (drawsContent 1)
-          (backgroundColor #00FF00)
         )
       )
     )
   )
 )