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

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef IntersectionObserver_h
 #define IntersectionObserver_h
 
 #include "bindings/core/v8/ScriptWrappable.h"
 #include "core/dom/Element.h"
 #include "core/dom/IntersectionObservation.h"
@@ skipping 14 matching lines @@
 
 public:
     static IntersectionObserver* create(const IntersectionObserverInit&, IntersectionObserverCallback&, ExceptionState&);
     static void resumeSuspendedObservers();
 
     // API methods
     void observe(Element*, ExceptionState&);
     void unobserve(Element*, ExceptionState&);
     HeapVector<Member<IntersectionObserverEntry>> takeRecords();
 
-    Element* root() { return m_root.get(); }
-    LayoutObject* rootLayoutObject();
+    Node* root() const { return m_root.get(); }
+    LayoutObject* rootLayoutObject() const;
     bool hasPercentMargin() const;
     const Length& topMargin() const { return m_topMargin; }
     const Length& rightMargin() const { return m_rightMargin; }
     const Length& bottomMargin() const { return m_bottomMargin; }
     const Length& leftMargin() const { return m_leftMargin; }
     bool isDescendantOfRoot(const Element*) const;
     void computeIntersectionObservations(double timestamp);
     void enqueueIntersectionObserverEntry(IntersectionObserverEntry&);
     void applyRootMargin(LayoutRect&) const;
     unsigned firstThresholdGreaterThan(float ratio) const;
     void deliver();
     void setActive(bool);
     void disconnect();
     void removeObservation(IntersectionObservation&);
 
     DECLARE_TRACE();
 
 private:
-    explicit IntersectionObserver(IntersectionObserverCallback&, Element&, const Vector<Length>& rootMargin, const Vector<float>& thresholds);
+    explicit IntersectionObserver(IntersectionObserverCallback&, Node&, const Vector<Length>& rootMargin, const Vector<float>& thresholds);
 
     void checkRootAndDetachIfNeeded();
 
     Member<IntersectionObserverCallback> m_callback;
-    WeakPtrWillBeWeakMember<Element> m_root;
+    WeakPtrWillBeWeakMember<Node> m_root;
     HeapHashSet<WeakMember<IntersectionObservation>> m_observations;
     HeapVector<Member<IntersectionObserverEntry>> m_entries;
     Vector<float> m_thresholds;
     Length m_topMargin;
     Length m_rightMargin;
     Length m_bottomMargin;
     Length m_leftMargin;
 };
 
 } // namespace blink
 
 #endif // IntersectionObserver_h