Use Shadow DOM to display fallback content for images

Source/core/html/HTMLElement.h

Index: Source/core/html/HTMLElement.h
diff --git a/Source/core/html/HTMLElement.h b/Source/core/html/HTMLElement.h
index 250b1ee7aea8b41939bf28dc89e3bc15e3f441e0..f91ea5e5df6ed2826c2ba6b47d1fbf90de9f7fa0 100644
--- a/Source/core/html/HTMLElement.h
+++ b/Source/core/html/HTMLElement.h
@@ -100,6 +100,10 @@ public:
     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);