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 9d53891b4ba691f8e80b675a33f3d4c5738664d1..4c240d8813ab59feef88bb3a4d7f58faf0634f42 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;
@@ -62,8 +64,16 @@ 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();
+    bool hasFallbackContent() const { return m_useFallbackContent; }
 
     IntPoint m_clickLocation; // Valid only during HTMLFormElement::prepareForSubmission().
+    bool m_useFallbackContent;
 };
 
 } // namespace blink