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 98ba2ff12dc2eb8ffadb26412fb7bb2fd96edd8c..753b8c22228a9145f85f16237606081cd146ed87 100644
--- a/Source/core/html/HTMLInputElement.h
+++ b/Source/core/html/HTMLInputElement.h
@@ -176,7 +176,7 @@ 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; }
 
@@ -271,6 +271,8 @@ public:
     AXObject* popupRootAXObject();
     virtual void didNotifySubtreeInsertionsToDocument() OVERRIDE;
 
+    virtual void ensureFallbackContent() OVERRIDE FINAL;
+    virtual void ensurePrimaryContent() OVERRIDE FINAL;
 protected:
     HTMLInputElement(Document&, HTMLFormElement*, bool createdByParser);