Avoid compositing fixed-position elements if they cannot scroll

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)
+      (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.