Use Shadow DOM to display fallback content for images

Source/core/rendering/RenderImage.h

Index: Source/core/rendering/RenderImage.h
diff --git a/Source/core/rendering/RenderImage.h b/Source/core/rendering/RenderImage.h
index 0ed41eab270abf0b164ccbd199f9b65f244222ac..99b81d3b9e5279b7f2b452c9cce1ff97958b322a 100644
--- a/Source/core/rendering/RenderImage.h
+++ b/Source/core/rendering/RenderImage.h
@@ -51,10 +51,6 @@ public:
     const RenderImageResource* imageResource() const { return m_imageResource.get(); }
     ImageResource* cachedImage() const { return m_imageResource ? m_imageResource->cachedImage() : 0; }
 
-    bool setImageSizeForAltText(ImageResource* newImage = 0);
-
-    void updateAltText();
-
     HTMLMapElement* imageMap() const;
     void areaElementFocusChanged(HTMLAreaElement*);
 
@@ -62,8 +58,6 @@ public:
 
     bool isGeneratedContent() const { return m_isGeneratedContent; }
 
-    String altText() const { return m_altText; }
-
     inline void setImageDevicePixelRatio(float factor) { m_imageDevicePixelRatio = factor; }
     float imageDevicePixelRatio() const { return m_imageDevicePixelRatio; }
 
@@ -106,14 +100,12 @@ private:
 
     virtual bool boxShadowShouldBeAppliedToBackground(BackgroundBleedAvoidance, InlineFlowBox*) const override final;
 
-    IntSize imageSizeForError(ImageResource*) const;
-    void paintInvalidationOrMarkForLayout(bool imageSizeChanged, const IntRect* = 0);
+    void repaintOrMarkForLayout(const IntRect* = 0);
     void updateIntrinsicSizeIfNeeded(const LayoutSize&);
     // Update the size of the image to be rendered. Object-fit may cause this to be different from the CSS box's content rect.
     void updateInnerContentRect();
 
     // Text to display as long as the image isn't available.
-    String m_altText;
     OwnPtr<RenderImageResource> m_imageResource;
     bool m_didIncrementVisuallyNonEmptyPixelCount;
     bool m_isGeneratedContent;