Avoid compositing fixed-position elements if they cannot scroll

LayoutTests/compositing/geometry/fixed-position-composited-switch-expected.txt

 Before (should be empty): 
 
 After (should not be empty): 
 (GraphicsLayer
-  (bounds 800.00 600.00)
+  (bounds 785.00 4016.00)
   (children 1
     (GraphicsLayer
-      (bounds 800.00 600.00)
+      (bounds 785.00 4016.00)
       (contentsOpaque 1)
       (children 1
         (GraphicsLayer
-          (position 495.00 30.00)
+          (position 480.00 30.00)
           (bounds 300.00 100.00)
           (contentsOpaque 1)
           (drawsContent 1)
           (backgroundColor #008000)
         )
       )
     )
   )
 )
 After (should be empty):