Remove NonCompositedContentHost -- Take 2

LayoutTests/compositing/layer-creation/fixed-position-nonscrollable-iframes-in-scrollable-page-expected.txt

 In all iframes, the green fixed-position element should not be composited.
 (GraphicsLayer
   (bounds 785.00 4016.00)
   (children 1
     (GraphicsLayer
       (bounds 785.00 4016.00)
       (contentsOpaque 1)
+      (drawsContent 1)
       (children 3
         (GraphicsLayer
           (position 50.00 360.00)
           (bounds 300.00 100.00)
           (contentsOpaque 1)
           (drawsContent 1)
           (backgroundColor #00FFFF)
         )
         (GraphicsLayer
           (position 10.00 200.00)
           (bounds 154.00 154.00)
           (drawsContent 1)
         )
         (GraphicsLayer
           (position 10.00 380.00)
           (bounds 154.00 154.00)
           (drawsContent 1)
         )
       )
     )
   )
 )
 Composited box underneath iframe.