ElementVisibilityObserver: get rid of reference cycle.

third_party/WebKit/Source/core/dom/IntersectionObserver.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 IntersectionObserver_h
 #define IntersectionObserver_h
 
 #include "bindings/core/v8/ExceptionStatePlaceholder.h"
 #include "bindings/core/v8/ScriptWrappable.h"
 #include "core/dom/IntersectionObservation.h"
@@ skipping 18 matching lines @@
 public:
     using EventCallback = Function<void(const HeapVector<Member<IntersectionObserverEntry>>&), WTF::SameThreadAffinity>;
 
     static IntersectionObserver* create(const IntersectionObserverInit&, IntersectionObserverCallback&, ExceptionState&);
     static IntersectionObserver* create(const Vector<Length>& rootMargin, const Vector<float>& thresholds, Document*, std::unique_ptr<EventCallback>);
     static void resumeSuspendedObservers();
 
     // API methods.
     void observe(Element*, ExceptionState& = ASSERT_NO_EXCEPTION);
     void unobserve(Element*, ExceptionState& = ASSERT_NO_EXCEPTION);
-    void disconnect(ExceptionState&);
+    void disconnect(ExceptionState& = ASSERT_NO_EXCEPTION);
     HeapVector<Member<IntersectionObserverEntry>> takeRecords(ExceptionState&);
 
     // API attributes.
     Element* root() const;
     String rootMargin() const;
     const Vector<float>& thresholds() const { return m_thresholds; }
 
     Node* rootNode() const { return m_root.get(); }
     LayoutObject* rootLayoutObject() const;
     const Length& topMargin() const { return m_topMargin; }
@@ skipping 22 matching lines @@
     Vector<float> m_thresholds;
     Length m_topMargin;
     Length m_rightMargin;
     Length m_bottomMargin;
     Length m_leftMargin;
 };
 
 } // namespace blink
 
 #endif // IntersectionObserver_h