Disable solid background color optimization for composited layers.

LayoutTests/compositing/layer-creation/fixed-position-change-out-of-view-in-view-expected.txt

 Layer tree when the fixed elements are out-of-view (should be blank):
 Layer tree when the fixed elements are in-view (both fixed elements should have layers):
 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (children 2
         (GraphicsLayer
           (position 100.00 50.00)
           (bounds 10.00 10.00)
           (contentsOpaque 1)
+          (drawsContent 1)
+          (backgroundColor #C0C0C0)
         )
         (GraphicsLayer
           (position 100.00 100.00)
           (bounds 10.00 10.00)
           (contentsOpaque 1)
+          (drawsContent 1)
+          (backgroundColor #C0C0C0)
         )
       )
     )
   )
 )
 Layer tree when the fixed elements are out-of-view again (should be blank):