Use Shadow DOM to display fallback content for images

Source/core/html/HTMLElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004-2007, 2009, 2014 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 82 matching lines @@
 
     virtual bool matchesReadOnlyPseudoClass() const override;
     virtual bool matchesReadWritePseudoClass() const override;
 
     static const AtomicString& eventParameterName();
 
     HTMLMenuElement* assignedContextMenu() const;
     HTMLMenuElement* contextMenu() const;
     void setContextMenu(HTMLMenuElement*);
 
+    virtual void ensureFallbackContent() { }

[pdr., 2014/11/14 07:34:17]

HTMLElement::ensureFallbackContent is too overloaded because there are several
variants of fallback content in the html spec (ruby, iframe, etc) that apply to
html elements. Can we move all of these back to HTMLImageElement and
HTMLInputElement? HTMLImageLoader's run function is the only place I see that a
conditional will be required.

I'm sorry this goes against a previous reviewer :(

+    virtual void ensurePrimaryContent() { }
+    virtual String altText() const { return String(); }
+
 protected:
     HTMLElement(const QualifiedName& tagName, Document&, ConstructionType);
 
     void addHTMLLengthToStyle(MutableStylePropertySet*, CSSPropertyID, const String& value);
     void addHTMLColorToStyle(MutableStylePropertySet*, CSSPropertyID, const String& color);
 
     void applyAlignmentAttributeToStyle(const AtomicString&, MutableStylePropertySet*);
     void applyBorderAttributeToStyle(const AtomicString&, MutableStylePropertySet*);
 
     virtual void parseAttribute(const QualifiedName&, const AtomicString&) override;
@@ skipping 61 matching lines @@
     template<typename T> inline bool is##thisType(const PassRefPtr<T>& node) { return is##thisType(node.get()); } \
     template<typename T> inline bool is##thisType(const RefPtr<T>& node) { return is##thisType(node.get()); } \
     template <> inline bool isElementOfType<const thisType>(const HTMLElement& element) { return is##thisType(element); } \
     DEFINE_ELEMENT_TYPE_CASTS_WITH_FUNCTION(thisType)
 
 } // namespace blink
 
 #include "core/HTMLElementTypeHelpers.h"
 
 #endif // HTMLElement_h