Disable solid background color optimization for composited layers.

LayoutTests/platform/chromium-win/compositing/geometry/limit-layer-bounds-overflow-root-expected.txt

 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (children 4
         (GraphicsLayer
           (position 21.00 21.00)
           (bounds 100.00 100.00)
         )
         (GraphicsLayer
           (position 21.00 21.00)
           (bounds 100.00 100.00)
           (contentsOpaque 1)
+          (drawsContent 1)
+          (backgroundColor #FF0000)
         )
         (GraphicsLayer
           (bounds 142.00 142.00)
           (drawsContent 1)
         )
         (GraphicsLayer
           (position 0.00 13.00)
           (bounds 216.00 16.00)
           (drawsContent 1)
         )
       )
     )
   )
 )