Disable solid background color optimization for composited layers.

LayoutTests/platform/chromium-win/compositing/backing/no-backing-for-clip-expected.txt

 This layer should not have backing store.
 This layer should not have backing store.
 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (children 1
         (GraphicsLayer
           (position 8.00 8.00)
           (bounds 342.00 244.00)
           (children 1
             (GraphicsLayer
               (position 1.00 1.00)
               (bounds 340.00 242.00)
               (children 1
                 (GraphicsLayer
                   (position 20.00 40.00)
                   (bounds 320.00 182.00)
                   (children 1
                     (GraphicsLayer
                       (position 1.00 1.00)
                       (bounds 340.00 180.00)
                       (children 1
                         (GraphicsLayer
                           (position 30.00 50.00)
                           (bounds 100.00 100.00)
                           (contentsOpaque 1)
+                          (drawsContent 1)
+                          (backgroundColor #C0C0C0)
                         )
                       )
                     )
                   )
                 )
               )
             )
           )
         )
       )
     )
   )
 )