Avoid compositing fixed-position elements if they cannot scroll

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

Index: LayoutTests/compositing/layer-creation/fixed-position-nonscrollable-body-mismatch-containers-expected.txt
diff --git a/LayoutTests/compositing/rtl/rtl-absolute-expected.txt b/LayoutTests/compositing/layer-creation/fixed-position-nonscrollable-body-mismatch-containers-expected.txt
similarity index 59%
copy from LayoutTests/compositing/rtl/rtl-absolute-expected.txt
copy to LayoutTests/compositing/layer-creation/fixed-position-nonscrollable-body-mismatch-containers-expected.txt
index 6e87875cdbe31e1d6e690afcab621d67d3a9e60b..324399a6d2d12787787b19de123bdfe7ad304284 100644
--- a/LayoutTests/compositing/rtl/rtl-absolute-expected.txt
+++ b/LayoutTests/compositing/layer-creation/fixed-position-nonscrollable-body-mismatch-containers-expected.txt
@@ -1,3 +1,5 @@
+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
@@ -6,11 +8,9 @@
       (contentsOpaque 1)
       (children 1
         (GraphicsLayer
-          (position 50.00 50.00)
+          (position 10.00 100.00)
           (bounds 100.00 100.00)
           (contentsOpaque 1)
-          (drawsContent 1)
-          (backgroundColor #008000)
         )
       )
     )