Avoid using instanceRoot in layout tests

LayoutTests/svg/custom/use-on-use-with-child-expected.txt

Index: LayoutTests/svg/custom/use-on-use-with-child-expected.txt
diff --git a/LayoutTests/svg/custom/use-on-use-with-child-expected.txt b/LayoutTests/svg/custom/use-on-use-with-child-expected.txt
index ee7ec66b51273129889b9c8a823aae6e741d2877..4eb5570b2da097fd39ab4057c4857cc0f31263e2 100644
--- a/LayoutTests/svg/custom/use-on-use-with-child-expected.txt
+++ b/LayoutTests/svg/custom/use-on-use-with-child-expected.txt
@@ -1,6 +1,10 @@
 The instance tree of use element zoomplus:
-
-     id = loupePlus  this = [object SVGElementInstance]  correspondingElement = [object SVGGElement]
-          id = useRim  this = [object SVGElementInstance]  correspondingElement = [object SVGUseElement]
-               id = rim  this = [object SVGElementInstance]  correspondingElement = [object SVGCircleElement]
+      this = [object ShadowRoot]
+           id = loupePlus this = [object SVGGElement]
+                this = [object Text]
+                id = useRim this = [object SVGGElement]
+                     this = [object Text]
+                     this = [object Text]
+                     id = rim this = [object SVGCircleElement]
+                this = [object Text]