Implement the ::backdrop pseudo-element for modal <dialog>.

Source/core/dom/PseudoElement.h

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Neither the name of Google Inc. nor the names of its
@@ skipping 49 matching lines @@
     virtual void didRecalcStyle(StyleChange) OVERRIDE;
     virtual PseudoId customPseudoId() const OVERRIDE { return m_pseudoId; }
 
     PseudoId m_pseudoId;
 };
 
 const QualifiedName& pseudoElementTagName();
 
 inline bool pseudoElementRendererIsNeeded(const RenderStyle* style)
 {
-    return style && style->display() != NONE && (style->contentData() || !style->regionThread().isEmpty());
+    return style && style->display() != NONE && (style->styleType() == BACKDROP || style->contentData() || !style->regionThread().isEmpty());
 }
 
 } // namespace
 
 #endif