Make multipart image documents work again.

Source/core/fetch/ImageResource.h

 /*
     Copyright (C) 1998 Lars Knoll (knoll@mpi-hd.mpg.de)
     Copyright (C) 2001 Dirk Mueller <mueller@kde.org>
     Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
     Copyright (C) 2004, 2005, 2006, 2007 Apple Inc. All rights reserved.
 
     This library is free software; you can redistribute it and/or
     modify it under the terms of the GNU Library General Public
     License as published by the Free Software Foundation; either
     version 2 of the License, or (at your option) any later version.
@@ skipping 110 matching lines @@
     void clear();
 
     void setCustomAcceptHeader();
     void createImage();
     void updateImage(bool allDataReceived);
     void clearImage();
     // If not null, changeRect is the changed part of the image.
     void notifyObservers(const IntRect* changeRect = nullptr);
     IntSize svgImageSizeForLayoutObject(const LayoutObject*) const;
     blink::Image* svgImageForLayoutObject(const LayoutObject*);
+    bool loadingMultipartContent() const;
 
     float m_devicePixelRatioHeaderValue;
 
     typedef HashMap<const ImageResourceClient*, RefPtr<SVGImageForContainer>> ImageForContainerMap;
     OwnPtr<ImageForContainerMap> m_imageForContainerMap;
 
     RefPtr<blink::Image> m_image;
-    bool m_loadingMultipartContent;
     bool m_hasDevicePixelRatioHeaderValue;
 };
 
 DEFINE_RESOURCE_TYPE_CASTS(Image);
 
 }
 
 #endif