IntersectionObserver: use nullptr for implicit root.

third_party/WebKit/Source/core/dom/NodeIntersectionObserverData.cpp

-// 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.
-
-#include "core/dom/NodeIntersectionObserverData.h"
-
-#include "core/dom/Document.h"
-#include "core/dom/IntersectionObservation.h"
-#include "core/dom/IntersectionObserver.h"
-#include "core/dom/IntersectionObserverController.h"
-
-namespace blink {
-
-NodeIntersectionObserverData::NodeIntersectionObserverData() {}
-
-IntersectionObservation* NodeIntersectionObserverData::getObservationFor(
-    IntersectionObserver& observer) {
-  auto i = m_intersectionObservations.find(&observer);
-  if (i == m_intersectionObservations.end())
-    return nullptr;
-  return i->value;
-}
-
-void NodeIntersectionObserverData::addObservation(
-    IntersectionObservation& observation) {
-  m_intersectionObservations.add(
-      TraceWrapperMember<IntersectionObserver>(this, &observation.observer()),
-      &observation);
-}
-
-void NodeIntersectionObserverData::removeObservation(
-    IntersectionObserver& observer) {
-  m_intersectionObservations.remove(&observer);
-}
-
-void NodeIntersectionObserverData::activateValidIntersectionObservers(
-    Node& node) {
-  IntersectionObserverController& controller =
-      node.document().ensureIntersectionObserverController();
-  for (auto& observer : m_intersectionObservers)
-    controller.addTrackedObserver(*observer);
-}
-
-void NodeIntersectionObserverData::deactivateAllIntersectionObservers(
-    Node& node) {
-  node.document()
-      .ensureIntersectionObserverController()
-      .removeTrackedObserversForRoot(node);
-}
-
-DEFINE_TRACE(NodeIntersectionObserverData) {
-  visitor->trace(m_intersectionObservers);
-  visitor->trace(m_intersectionObservations);
-}
-
-DEFINE_TRACE_WRAPPERS(NodeIntersectionObserverData) {
-  for (auto& entry : m_intersectionObservations) {
-    visitor->traceWrappers(entry.key);
-  }
-}
-
-}  // namespace blink