Use Shadow DOM to display fallback content for images

Source/core/html/HTMLImageFallbackHelper.h

Index: Source/core/html/HTMLImageFallbackHelper.h
diff --git a/Source/core/html/HTMLImageFallbackHelper.h b/Source/core/html/HTMLImageFallbackHelper.h
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+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef HTMLImageFallbackHelper_h
+#define HTMLImageFallbackHelper_h
+
+#include "wtf/PassRefPtr.h"
+
+namespace blink {
+
+class Element;
+class RenderStyle;
+
+class HTMLImageFallbackHelper {
+public:
+    static void createAltTextShadowTree(Element&);
+    static PassRefPtr<RenderStyle> customStyleForAltText(Element&, PassRefPtr<RenderStyle> newStyle);
+};
+
+} // namespace blink
+
+#endif // HTMLImageFallbackHelper_h