Re-enable solid background color optimization for composited layers.

LayoutTests/virtual/gpu/compositedscrolling/overflow/composited-scrolling-creates-a-stacking-container-expected.txt

 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (drawsContent 1)
       (children 1
         (GraphicsLayer
           (position 28.00 20.00)
           (bounds 202.00 202.00)
           (drawsContent 1)
           (children 4
             (GraphicsLayer
               (position 1.00 1.00)
               (bounds 185.00 185.00)
               (children 1
                 (GraphicsLayer
                   (bounds 200.00 300.00)
                   (drawsContent 1)
                   (children 2
                     (GraphicsLayer
                       (bounds 200.00 300.00)
                       (drawsContent 1)
                     )
                     (GraphicsLayer
                       (bounds 200.00 150.00)
                       (contentsOpaque 1)
-                      (drawsContent 1)
-                      (backgroundColor #008000)
                     )
                   )
                 )
               )
             )
             (GraphicsLayer
               (position 1.00 186.00)
               (bounds 185.00 15.00)
               (drawsContent 1)
             )
             (GraphicsLayer
               (position 186.00 1.00)
               (bounds 15.00 185.00)
               (drawsContent 1)
             )
             (GraphicsLayer
               (position 186.00 186.00)
               (bounds 15.00 15.00)
               (drawsContent 1)
             )
           )
         )
       )
     )
   )
 )