IntersectionObserver: use nullptr for implicit root.

third_party/WebKit/Source/core/dom/ElementIntersectionObserverData.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 NodeIntersectionObserverData_h
-#define NodeIntersectionObserverData_h
+#ifndef ElementIntersectionObserverData_h
+#define ElementIntersectionObserverData_h
 
 #include "bindings/core/v8/ScriptWrappable.h"
 #include "bindings/core/v8/TraceWrapperMember.h"
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class Node;
 class IntersectionObservation;
 class IntersectionObserver;
 
-class NodeIntersectionObserverData
-    : public GarbageCollected<NodeIntersectionObserverData>,
+class ElementIntersectionObserverData
+    : public GarbageCollected<ElementIntersectionObserverData>,
       public TraceWrapperBase {
  public:
-  NodeIntersectionObserverData();
+  ElementIntersectionObserverData();
 
   IntersectionObservation* getObservationFor(IntersectionObserver&);
+  void addObserver(IntersectionObserver&);
+  void removeObserver(IntersectionObserver&);
   void addObservation(IntersectionObservation&);
   void removeObservation(IntersectionObserver&);
   void activateValidIntersectionObservers(Node&);
   void deactivateAllIntersectionObservers(Node&);
 
   DECLARE_TRACE();
   DECLARE_TRACE_WRAPPERS();
 
  private:
   // IntersectionObservers for which the Node owning this data is root.
   HeapHashSet<WeakMember<IntersectionObserver>> m_intersectionObservers;
   // IntersectionObservations for which the Node owning this data is target.
   HeapHashMap<TraceWrapperMember<IntersectionObserver>,
               Member<IntersectionObservation>>
       m_intersectionObservations;
 };
 
 }  // namespace blink
 
-#endif  // NodeIntersectionObserverData_h
+#endif  // ElementIntersectionObserverData_h