Use Shadow DOM to display fallback content for images

Source/core/html/HTMLInputElement.h

Index: Source/core/html/HTMLInputElement.h
diff --git a/Source/core/html/HTMLInputElement.h b/Source/core/html/HTMLInputElement.h
index bc4f31fe6913e31ef673f47fe758e870e57ae08e..cfc4ba3dd3a6625cd584a1cb1ce9aa884a43521c 100644
--- a/Source/core/html/HTMLInputElement.h
+++ b/Source/core/html/HTMLInputElement.h
@@ -154,7 +154,7 @@ public:
     virtual bool isActivatedSubmit() const override final;
     virtual void setActivatedSubmit(bool flag) override final;
 
-    String altText() const;
+    virtual String altText() const override final;
 
     int maxResults() const { return m_maxResults; }
 
@@ -252,6 +252,9 @@ public:
     AXObject* popupRootAXObject();
     virtual void didNotifySubtreeInsertionsToDocument() override;
 
+    virtual void ensureFallbackContent() override final;
+    virtual void ensurePrimaryContent() override final;
+    bool hasFallbackContent() const;
 protected:
     HTMLInputElement(Document&, HTMLFormElement*, bool createdByParser);
 
@@ -348,9 +351,7 @@ private:
     RadioButtonGroupScope* radioButtonGroupScope() const;
     void addToRadioButtonGroup();
     void removeFromRadioButtonGroup();
-#if ENABLE(INPUT_MULTIPLE_FIELDS_UI)
     virtual PassRefPtr<RenderStyle> customStyleForRenderer() override;
-#endif
 
     virtual bool shouldDispatchFormControlChangeEvent(String&, String&) override;