Re-land "Drop background color optimization for composited layers"

LayoutTests/compositing/iframes/connect-compositing-iframe-delayed-expected.txt

 
 When the parent document becomes composited, the layer trees should get connected together.
 
 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (drawsContent 1)
@@ skipping 15 matching lines @@
                           (bounds 285.00 230.00)
                           (children 1
                             (GraphicsLayer
                               (bounds 285.00 230.00)
                               (drawsContent 1)
                               (children 1
                                 (GraphicsLayer
                                   (position 18.00 10.00)
                                   (bounds 210.00 210.00)
                                   (contentsOpaque 1)
+                                  (drawsContent 1)
+                                  (backgroundColor #0000FF)
                                 )
                               )
                             )
                           )
                         )
                       )
                     )
                   )
                 )
                 (GraphicsLayer
                   (position 285.00 0.00)
                   (bounds 15.00 150.00)
                   (drawsContent 1)
                 )
               )
             )
           )
         )
         (GraphicsLayer
           (position 8.00 8.00)
           (bounds 100.00 100.00)
           (contentsOpaque 1)
+          (drawsContent 1)
+          (backgroundColor #0000FF)
         )
       )
     )
   )
 )