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

Source/core/dom/PseudoElement.cpp

 /*
  * 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 63 matching lines @@
     return parentOrShadowHostElement()->renderer()->getCachedPseudoStyle(m_pseudoId);
 }
 
 void PseudoElement::attach(const AttachContext& context)
 {
     ASSERT(!renderer());
 
     Element::attach(context);
 
     RenderObject* renderer = this->renderer();
-    if (!renderer || !renderer->style()->regionThread().isEmpty())
+    if (!renderer || !renderer->style()->regionThread().isEmpty() || renderer->style()->styleType() == BACKDROP)
         return;
 
     RenderStyle* style = renderer->style();
     ASSERT(style->contentData());
 
     for (const ContentData* content = style->contentData(); content; content = content->next()) {
         RenderObject* child = content->createRenderer(document(), style);
         if (renderer->isChildAllowed(child, style)) {
             renderer->addChild(child);
             if (child->isQuote())
@@ skipping 23 matching lines @@
 
         // The style for the RenderTextFragment for first letter is managed by an enclosing block, not by us.
         if (child->style()->styleType() == FIRST_LETTER)
             continue;
 
         child->setPseudoStyle(renderer->style());
     }
 }
 
 } // namespace