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 33adcf0adde82c3bdc3609b9b85c75bc784c940e..c602f913168b3b87b8a8f42f409e48d961ceb4a8 100644
--- a/Source/core/rendering/RenderImage.h
+++ b/Source/core/rendering/RenderImage.h
@@ -47,10 +47,6 @@ 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*);
 
@@ -60,8 +56,6 @@ 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; }
 
@@ -101,8 +95,7 @@ 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();
@@ -110,7 +103,6 @@ private:
     void paintAreaElementFocusRing(PaintInfo&);
 
     // Text to display as long as the image isn't available.
-    String m_altText;
     OwnPtr<RenderImageResource> m_imageResource;
     bool m_didIncrementVisuallyNonEmptyPixelCount;
     bool m_isGeneratedContent;