Disable solid background color optimization for composited layers.

LayoutTests/compositing/repaint/invalidations-on-composited-layers-expected.txt

 (repaint rects
   (rect 8 13 784 16)
   (rect 8 413 784 29)
   (rect 0 421 800 29)
 )
 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (children 1
         (GraphicsLayer
           (position 8.00 42.00)
           (bounds 400.00 400.00)
           (contentsOpaque 1)
+          (drawsContent 1)
+          (backgroundColor #008000)
+          (repaint rects
+            (rect 0.00 0.00 400.00 400.00)
+            (rect 0.00 0.00 400.00 400.00)
+            (rect 0.00 0.00 400.00 400.00)
+          )
           (children 1
             (GraphicsLayer
               (position 50.00 50.00)
               (bounds 75.00 75.00)
               (contentsOpaque 1)
+              (drawsContent 1)
+              (backgroundColor #0000FF)
+              (repaint rects
+                (rect 0.00 0.00 75.00 75.00)
+                (rect 0.00 0.00 75.00 75.00)
+                (rect 0.00 0.00 75.00 75.00)
+              )
             )
           )
         )
       )
     )
   )
 )