Re-enable solid background color optimization for composited layers.

LayoutTests/compositing/overflow/opt-in-all-layers-after-unclipped-descendant-expected.txt

 This test ensures elements coming after an unclipped sibling in stacking order will get promoted. This is necessary because otherwise the element will render behind the unclipped sibling at first, until overlap is checked.
 
 (GraphicsLayer
   (bounds 1784.00 722.00)
   (children 1
     (GraphicsLayer
       (bounds 1784.00 722.00)
       (contentsOpaque 1)
       (drawsContent 1)
       (children 1
         (GraphicsLayer
           (position 28.00 320.00)
           (bounds 402.00 402.00)
           (drawsContent 1)
           (children 4
             (GraphicsLayer
               (position 1.00 1.00)
               (bounds 385.00 385.00)
               (children 1
                 (GraphicsLayer
                   (bounds 385.00 1250.00)
                   (drawsContent 1)
                   (children 4
                     (GraphicsLayer
                       (position 0.00 330.00)
                       (bounds 100.00 100.00)
                       (contentsOpaque 1)
-                      (drawsContent 1)
-                      (backgroundColor #0000FF)
                     )
                     (GraphicsLayer
                       (position 10.00 630.00)
                       (bounds 180.00 300.00)
                       (contentsOpaque 1)
-                      (drawsContent 1)
-                      (backgroundColor #808080)
                     )
                     (GraphicsLayer
                       (position 10.00 950.00)
                       (bounds 180.00 300.00)
                       (contentsOpaque 1)
-                      (drawsContent 1)
-                      (backgroundColor #808080)
                     )
                     (GraphicsLayer
                       (position 10.00 10.00)
                       (bounds 180.00 300.00)
                       (contentsOpaque 1)
-                      (drawsContent 1)
-                      (backgroundColor #808080)
                     )
                   )
                 )
               )
             )
             (GraphicsLayer
               (position 1.00 386.00)
               (bounds 385.00 15.00)
               (drawsContent 1)
             )
             (GraphicsLayer
               (position 386.00 1.00)
               (bounds 15.00 385.00)
               (drawsContent 1)
             )
             (GraphicsLayer
               (position 386.00 386.00)
               (bounds 15.00 15.00)
               (drawsContent 1)
             )
           )
         )
       )
     )
   )
 )