Avoid compositing fixed-position elements if they cannot scroll

LayoutTests/compositing/layer-creation/fixed-position-nonscrollable-body-mismatch-containers-expected.txt

+Even though the fixed-position element's container is nonscrollable, it should still be composited because one of its ancestors is scrolling.
+
 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (children 1
         (GraphicsLayer
-          (position 8.00 8.00)
+          (position 10.00 100.00)
           (bounds 100.00 100.00)
-          (opacity 0.50)
           (contentsOpaque 1)
           (drawsContent 1)
-          (backgroundColor #008000)
+          (backgroundColor #00FF00)
         )
       )
     )
   )
 )