Re-enable solid background color optimization for composited layers.

LayoutTests/platform/win/compositing/geometry/preserve-3d-switching-expected.txt

 The green box appear angled out from the yellow box and embedded in it.
 
 -webkit-transform: translateZ(-100px) rotateY(45deg);
 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (drawsContent 1)
       (children 1
         (GraphicsLayer
           (preserves3D 1)
           (transform [1.00 0.00 0.00 0.00] [0.00 1.00 0.00 0.00] [-0.52 -0.45 1.00 -0.00] [0.00 0.00 0.00 1.00])
           (children 1
             (GraphicsLayer
               (position 120.00 84.00)
               (bounds 280.00 280.00)
               (opacity 0.80)
               (contentsOpaque 1)
               (preserves3D 1)
-              (drawsContent 1)
-              (backgroundColor #FFFF00)
               (transform [0.77 -0.56 -0.32 0.00] [0.00 0.50 -0.87 0.00] [0.64 0.66 0.38 0.00] [0.00 0.00 0.00 1.00])
               (children 1
                 (GraphicsLayer
                   (position 40.00 40.00)
                   (bounds 200.00 200.00)
                   (opacity 0.70)
                   (contentsOpaque 1)
                   (drawsContent 1)
                   (backgroundColor #008000)
                   (transform [0.77 0.00 0.64 0.00] [0.00 1.00 0.00 0.00] [-0.64 0.00 0.77 0.00] [0.00 0.00 50.00 1.00])
                 )
               )
             )
           )
         )
       )
     )
   )
 )