Remove the concept of document.documentElement

sky/tests/clipping/background-opaque-clipped-gradients-expected.txt

Index: sky/tests/clipping/background-opaque-clipped-gradients-expected.txt
diff --git a/sky/tests/clipping/background-opaque-clipped-gradients-expected.txt b/sky/tests/clipping/background-opaque-clipped-gradients-expected.txt
index 0f6aeeba80f4a467fb12e7e3a1373e079abdb332..b127eb99028cefda0ea5d012a072d090893f41aa 100644
--- a/sky/tests/clipping/background-opaque-clipped-gradients-expected.txt
+++ b/sky/tests/clipping/background-opaque-clipped-gradients-expected.txt
@@ -1,10 +1,9 @@
 layer at (0,0) size 800x600
   RenderView {#document} at (0,0) size 800x600
-layer at (0,0) size 800x238
-  RenderBlock {sky} at (0,0) size 800x238
-    RenderParagraph (anonymous) at (0,0) size 800x38
-      RenderText {#text} at (0,0) size 778x38
-        text run at (0,0) width 778: "Test passes if the image below shows nested green, blue and yellow squares with a dotted black"
-        text run at (0,19) width 59: "border."
-    RenderBlock {div} at (0,38) size 200x200 [border: (25px dotted #000000)]
+    RenderBlock {sky} at (0,0) size 800x238
+      RenderParagraph (anonymous) at (0,0) size 800x38
+        RenderText {#text} at (0,0) size 778x38
+          text run at (0,0) width 778: "Test passes if the image below shows nested green, blue and yellow squares with a dotted black"
+          text run at (0,19) width 59: "border."
+      RenderBlock {div} at (0,38) size 200x200 [border: (25px dotted #000000)]