Use Shadow DOM to display fallback content for images

Source/core/html/HTMLImageElement.h

Index: Source/core/html/HTMLImageElement.h
diff --git a/Source/core/html/HTMLImageElement.h b/Source/core/html/HTMLImageElement.h
index 7ce5c3c37e5ba4c661109d0f00df43f8b7f19986..81d4ae1964b0f6b3a9d31f917a2a0ab449c46ac7 100644
--- a/Source/core/html/HTMLImageElement.h
+++ b/Source/core/html/HTMLImageElement.h
@@ -34,6 +34,7 @@ namespace blink {
 
 class HTMLFormElement;
 class ImageCandidate;
+class ShadowRoot;
 
 class HTMLImageElement FINAL : public HTMLElement, public CanvasImageSource {
     DEFINE_WRAPPERTYPEINFO();
@@ -56,7 +57,7 @@ public:
 
     bool isServerMap() const;
 
-    const AtomicString& altText() const;
+    virtual String altText() const OVERRIDE FINAL;
 
     CompositeOperator compositeOperator() const { return m_compositeOperator; }
 
@@ -88,6 +89,8 @@ public:
 
     virtual HTMLFormElement* formOwner() const OVERRIDE;
     void formRemovedFromTree(const Node& formRoot);
+    virtual void ensureFallbackContent() OVERRIDE FINAL;
+    virtual void ensurePrimaryContent() OVERRIDE FINAL;
 
     // CanvasImageSourceImplementations
     virtual PassRefPtr<Image> getSourceImageForCanvas(SourceImageMode, SourceImageStatus*) const;
@@ -98,11 +101,18 @@ public:
 
     // public so that HTMLPictureElement can call this as well.
     void selectSourceURL(ImageLoader::UpdateFromElementBehavior);
+    void reattachFallbackContent();
+    void setUseFallbackContent() { m_useFallbackContent = true; ensureUserAgentShadowRoot(); }
+    void setIsFallbackImage() { m_isFallbackImage = true; }
+
 protected:
     explicit HTMLImageElement(Document&, HTMLFormElement* = 0, bool createdByParser = false);
 
     virtual void didMoveToNewDocument(Document& oldDocument) OVERRIDE;
+    virtual bool useFallbackContent() const { return m_useFallbackContent; }
 
+    virtual void didAddUserAgentShadowRoot(ShadowRoot&) OVERRIDE;
+    virtual PassRefPtr<RenderStyle> customStyleForRenderer() OVERRIDE;
 private:
     virtual bool areAuthorShadowsAllowed() const OVERRIDE { return false; }
 
@@ -151,6 +161,8 @@ private:
     unsigned m_intrinsicSizingViewportDependant : 1;
     // Effective size is viewport dependant if the sizes attribute's effective size used v* length units.
     unsigned m_effectiveSizeViewportDependant : 1;
+    unsigned m_useFallbackContent : 1;
+    unsigned m_isFallbackImage : 1;
 };
 
 } // namespace blink