compositor-worker: Introduce CompositorProxy::disconnect().

Source/core/dom/CompositorProxy.h

 // Copyright 2015 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 CompositorProxy_h
 #define CompositorProxy_h
 
 #include "bindings/core/v8/ScriptWrappable.h"
 #include "core/dom/DOMMatrix.h"
 #include "core/dom/Element.h"
@@ skipping 27 matching lines @@
 
     DEFINE_INLINE_TRACE()
     {
         visitor->trace(m_transform);
     }
 
     uint64_t elementId() const { return m_elementId; }
     uint32_t bitfieldsSupported() const { return m_bitfieldsSupported; }
     bool supports(const String& attribute) const;
 
+    bool connected() const { return m_connected; }
+    void disconnect();
+
     double opacity(ExceptionState&) const;
     double scrollLeft(ExceptionState&) const;
     double scrollTop(ExceptionState&) const;
     DOMMatrix* transform(ExceptionState&) const;
 
     void setOpacity(double, ExceptionState&);
     void setScrollLeft(double, ExceptionState&);
     void setScrollTop(double, ExceptionState&);
     void setTransform(DOMMatrix*, ExceptionState&);
 
 protected:
     CompositorProxy(Element&, const Vector<String>& attributeArray);
     CompositorProxy(uint64_t element, uint32_t attributeFlags);
 
 private:
     bool raiseExceptionIfNotMutable(Attributes, ExceptionState&) const;
 
     const uint64_t m_elementId = 0;
     const uint32_t m_bitfieldsSupported = 0;
     uint32_t m_mutatedAttributes = 0;
 
     double m_opacity = 0;
     double m_scrollLeft = 0;
     double m_scrollTop = 0;
     Member<DOMMatrix> m_transform;
+
+    bool m_connected = true;
 };
 
 } // namespace blink
 
 #endif // CompositorProxy_h