Use registerWeakMembers to clean up IntersectionObserver.

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

Index: third_party/WebKit/Source/core/dom/IntersectionObserver.h
diff --git a/third_party/WebKit/Source/core/dom/IntersectionObserver.h b/third_party/WebKit/Source/core/dom/IntersectionObserver.h
index f793a6f958543add45b3d8b5810b6d6c2297357d..a918acb4cab2f8f6a182a4fbfbeecd9ebbc6672f 100644
--- a/third_party/WebKit/Source/core/dom/IntersectionObserver.h
+++ b/third_party/WebKit/Source/core/dom/IntersectionObserver.h
@@ -27,6 +27,7 @@ class IntersectionObserver final : public GarbageCollectedFinalized<Intersection
 public:
     static IntersectionObserver* create(const IntersectionObserverInit&, IntersectionObserverCallback&, ExceptionState&);
     static void resumeSuspendedObservers();
+    static void rootDisappearedCallback(Visitor*, void*);
 
     // API methods
     void observe(Element*, ExceptionState&);
@@ -56,8 +57,6 @@ public:
 private:
     explicit IntersectionObserver(IntersectionObserverCallback&, Node&, const Vector<Length>& rootMargin, const Vector<float>& thresholds);
 
-    void checkRootAndDetachIfNeeded();
-
     Member<IntersectionObserverCallback> m_callback;
     WeakPtrWillBeWeakMember<Node> m_root;
     HeapHashSet<WeakMember<IntersectionObservation>> m_observations;