Use Document, rather than document element, for implicit root.

third_party/WebKit/Source/core/dom/Element.h

Index: third_party/WebKit/Source/core/dom/Element.h
diff --git a/third_party/WebKit/Source/core/dom/Element.h b/third_party/WebKit/Source/core/dom/Element.h
index e778e157399415c785276cfeb2dad5f61d37dc84..63839905bfc23d40355d803d55e566aad49b78a3 100644
--- a/third_party/WebKit/Source/core/dom/Element.h
+++ b/third_party/WebKit/Source/core/dom/Element.h
@@ -52,7 +52,6 @@ class CustomElementDefinition;
 class DOMStringMap;
 class DOMTokenList;
 class Dictionary;
-class ElementIntersectionObserverData;
 class ElementRareData;
 class ElementShadow;
 class ExceptionState;
@@ -60,6 +59,7 @@ class Image;
 class IntSize;
 class Locale;
 class MutableStylePropertySet;
+class NodeIntersectionObserverData;
 class PropertySetCSSStyleDeclaration;
 class PseudoElement;
 class ScrollState;
@@ -554,8 +554,8 @@ public:
 
     SpellcheckAttributeState spellcheckAttributeState() const;
 
-    ElementIntersectionObserverData* intersectionObserverData() const;
-    ElementIntersectionObserverData& ensureIntersectionObserverData();
+    NodeIntersectionObserverData* intersectionObserverData() const;
+    NodeIntersectionObserverData& ensureIntersectionObserverData();
 
 protected:
     Element(const QualifiedName& tagName, Document*, ConstructionType);