Use Shadow DOM to display fallback content for images

Source/core/loader/ImageLoader.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2009 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 106 matching lines @@
     virtual void notifyFinished(Resource*) OVERRIDE;
 
 private:
     class Task;
 
     // Called from the task or from updateFromElement to initiate the load.
     void doUpdateFromElement(BypassMainWorldBehavior, UpdateFromElementBehavior);
 
     virtual void dispatchLoadEvent() = 0;
     virtual String sourceURI(const AtomicString&) const = 0;
+    virtual void noImageResourceToLoad() { };
 
     void updatedHasPendingEvent();
 
     void dispatchPendingLoadEvent();
     void dispatchPendingErrorEvent();
 
     RenderImageResource* renderImageResource();
     void updateRenderer();
 
     void setImageWithoutConsideringPendingLoadEvent(ImageResource*);
@@ skipping 48 matching lines @@
     bool m_hasPendingErrorEvent : 1;
     bool m_imageComplete : 1;
     bool m_loadingImageDocument : 1;
     bool m_elementIsProtected : 1;
     unsigned m_highPriorityClientCount;
 };
 
 }
 
 #endif