Use Shadow DOM to display fallback content for images

Source/core/html/forms/ImageInputType.h

Index: Source/core/html/forms/ImageInputType.h
diff --git a/Source/core/html/forms/ImageInputType.h b/Source/core/html/forms/ImageInputType.h
index 122bd2c469f0a601692024a43b4dc75da0306299..e6157d0a76542230d4b3b2bc40316c6492197a4f 100644
--- a/Source/core/html/forms/ImageInputType.h
+++ b/Source/core/html/forms/ImageInputType.h
@@ -41,6 +41,7 @@ namespace blink {
 class ImageInputType FINAL : public BaseButtonInputType {
 public:
     static PassRefPtrWillBeRawPtr<InputType> create(HTMLInputElement&);
+    virtual PassRefPtr<RenderStyle> customStyleForRenderer(PassRefPtr<RenderStyle>);
 
 private:
     ImageInputType(HTMLInputElement&);
@@ -53,6 +54,7 @@ private:
     virtual void handleDOMActivateEvent(Event*) OVERRIDE;
     virtual void altAttributeChanged() OVERRIDE;
     virtual void srcAttributeChanged() OVERRIDE;
+    virtual void valueAttributeChanged() OVERRIDE;
     virtual void startResourceLoading() OVERRIDE;
     virtual bool shouldRespectAlignAttribute() OVERRIDE;
     virtual bool canBeSuccessfulSubmitButton() OVERRIDE;
@@ -63,8 +65,17 @@ private:
     virtual unsigned width() const OVERRIDE;
     virtual bool hasLegalLinkAttribute(const QualifiedName&) const OVERRIDE;
     virtual const QualifiedName& subResourceAttributeName() const OVERRIDE;
+    virtual void ensureFallbackContent() OVERRIDE;
+    virtual void ensurePrimaryContent() OVERRIDE;
+    virtual void createShadowSubtree() OVERRIDE;
+
+    void reattachFallbackContent();
+    void setUseFallbackContent();
+    Text* altTextNode() { return m_altTextNode; }
 
     IntPoint m_clickLocation; // Valid only during HTMLFormElement::prepareForSubmission().
+    bool m_useFallbackContent;
+    Text* m_altTextNode;
 };
 
 } // namespace blink