IntersectionObserver: use nullptr for implicit root.

third_party/WebKit/Source/core/dom/IntersectionObserverController.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/IntersectionObserverController.h"
 
 #include "core/dom/Document.h"
+#include "core/dom/Element.h"
 #include "core/dom/TaskRunnerHelper.h"
 #include "platform/tracing/TraceEvent.h"
 
 namespace blink {
 
 IntersectionObserverController* IntersectionObserverController::create(
     Document* document) {
   IntersectionObserverController* result =
       new IntersectionObserverController(document);
   result->suspendIfNeeded();
@@ skipping 61 matching lines @@
 
 void IntersectionObserverController::addTrackedObserver(
     IntersectionObserver& observer) {
   m_trackedIntersectionObservers.add(&observer);
 }
 
 void IntersectionObserverController::removeTrackedObserversForRoot(
     const Node& root) {
   HeapVector<Member<IntersectionObserver>> toRemove;
   for (auto& observer : m_trackedIntersectionObservers) {
-    if (observer->rootNode() == &root)
+    if (observer->root() == &root)
       toRemove.append(observer);
   }
   m_trackedIntersectionObservers.removeAll(toRemove);
 }
 
 DEFINE_TRACE(IntersectionObserverController) {
   visitor->trace(m_trackedIntersectionObservers);
   visitor->trace(m_pendingIntersectionObservers);
   SuspendableObject::trace(visitor);
 }
 
 }  // namespace blink