IntersectionObserver: use nullptr for implicit root.

third_party/WebKit/Source/core/dom/NodeIntersectionObserverData.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
-
-#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>,
-      public TraceWrapperBase {
- public:
-  NodeIntersectionObserverData();
-
-  IntersectionObservation* getObservationFor(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