Use Document, rather than document element, for implicit 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"
 
 namespace blink {
 
 class Element;
 class IntersectionObserver;
+class Node;
 
 class IntersectionObservation : public GarbageCollectedFinalized<IntersectionObservation> {
 public:
     IntersectionObservation(IntersectionObserver&, Element&, bool shouldReportRootBounds);
 
     struct IntersectionGeometry {
         LayoutRect targetRect;
         LayoutRect intersectionRect;
         LayoutRect rootRect;
     };
 
     IntersectionObserver& observer() const { return *m_observer; }
-    Element* target() const { return m_target.get(); }
+    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();
 
     DECLARE_TRACE();
 
 private:
     void initializeGeometry(IntersectionGeometry&);
     void clipToRoot(LayoutRect&);
     void clipToFrameView(IntersectionGeometry&);
     bool computeGeometry(IntersectionGeometry&);
 
     Member<IntersectionObserver> m_observer;
-    WeakPtrWillBeWeakMember<Element> m_target;
+    WeakPtrWillBeWeakMember<Node> m_target;

[ojan, 2016/01/12 01:28:18]

Isn't target still always an Element?

[szager1, 2016/01/12 21:19:51]

Yeah, this weird, but this comes from
NodeIntersectionObserverData::createWeakPtr(), which calls
WeakPointerFactory<Node>::createWeakPtr(), which returns a WeakPtr<Node>, which
cannot be coerced into a WeakPtr<Element> (aside from static_cast, which I want
to avoid at all costs).

When oilpan rolls out, this can become a WeakMember<Element>.

Added a comment and a TODO.

     unsigned m_active : 1;
     unsigned m_shouldReportRootBounds : 1;
     unsigned m_lastThresholdIndex : 30;
 };
 
 } // namespace blink
 
 #endif // IntersectionObservation_h