Use Document, rather than document element, for implicit root.

third_party/WebKit/Source/core/dom/IntersectionObservation.h

Index: third_party/WebKit/Source/core/dom/IntersectionObservation.h
diff --git a/third_party/WebKit/Source/core/dom/IntersectionObservation.h b/third_party/WebKit/Source/core/dom/IntersectionObservation.h
index e832c6213f2c2bae809479537a4c3a587e07aaff..63b9a464641294104f7848fc4994caadfdaf847b 100644
--- a/third_party/WebKit/Source/core/dom/IntersectionObservation.h
+++ b/third_party/WebKit/Source/core/dom/IntersectionObservation.h
@@ -12,6 +12,7 @@ namespace blink {
 
 class Element;
 class IntersectionObserver;
+class Node;
 
 class IntersectionObservation : public GarbageCollectedFinalized<IntersectionObservation> {
 public:
@@ -24,7 +25,7 @@ public:
     };
 
     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; }
@@ -43,7 +44,7 @@ private:
     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;