Use registerWeakMembers to manage observer root.

third_party/WebKit/Source/core/dom/IntersectionObservation.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 IntersectionObservation_h
 #define IntersectionObservation_h
 
 #include "platform/geometry/LayoutRect.h"
 #include "platform/heap/Handle.h"
 
@@ skipping 14 matching lines @@
         LayoutRect rootRect;
     };
 
     IntersectionObserver& observer() const { return *m_observer; }
     Element* target() const;
     bool isActive() const { return m_active; }
     void setActive(bool active) { m_active = active; }
     unsigned lastThresholdIndex() const { return m_lastThresholdIndex; }
     void setLastThresholdIndex(unsigned index) { m_lastThresholdIndex = index; }
     bool shouldReportRootBounds() const { return m_shouldReportRootBounds; }
-
     void computeIntersectionObservations(double timestamp);
     void disconnect();
+    void clearRootAndRemoveFromTarget();
 
     DECLARE_TRACE();
 
 private:
     void initializeGeometry(IntersectionGeometry&);
     void clipToRoot(LayoutRect&);
     void clipToFrameView(IntersectionGeometry&);
     bool computeGeometry(IntersectionGeometry&);
 
     Member<IntersectionObserver> m_observer;
 
     // TODO(szager): Why Node instead of Element?  Because NodeIntersectionObserverData::createWeakPtr()
     // returns a WeakPtr<Node>, which cannot be coerced into a WeakPtr<Element>.  When oilpan rolls out,
     // this can be changed to WeakMember<Element>.
     WeakPtrWillBeWeakMember<Node> m_target;
 
     unsigned m_active : 1;
     unsigned m_shouldReportRootBounds : 1;
     unsigned m_lastThresholdIndex : 30;
 };
 
 } // namespace blink
 
 #endif // IntersectionObservation_h