Add a runtime flag in WebRuntimeFeatures to enable 'image/webp' accept header

Source/WebCore/loader/cache/CachedImage.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 81 matching lines @@
 
     virtual bool shouldPauseAnimation(const Image*);
     virtual void animationAdvanced(const Image*);
     virtual void changedInRect(const Image*, const IntRect&);
 
     virtual void reportMemoryUsage(MemoryObjectInfo*) const OVERRIDE;
 
 private:
     void clear();
 
+    void setCustomAcceptHeader();
     void createImage();
     void clearImage();
     // If not null, changeRect is the changed part of the image.
     void notifyObservers(const IntRect* changeRect = 0);
     virtual PurgePriority purgePriority() const { return PurgeFirst; }
 
     virtual void switchClientsToRevalidatedResource() OVERRIDE;
 
     typedef pair<IntSize, float> SizeAndZoom;
     typedef HashMap<const CachedImageClient*, SizeAndZoom> ContainerSizeRequests;
     ContainerSizeRequests m_pendingContainerSizeRequests;
 
     RefPtr<Image> m_image;
 #if ENABLE(SVG)
     OwnPtr<SVGImageCache> m_svgImageCache;
 #endif
 };
 
 }
 
 #endif