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

Source/core/dom/PseudoElement.cpp

Index: Source/core/dom/PseudoElement.cpp
diff --git a/Source/core/dom/PseudoElement.cpp b/Source/core/dom/PseudoElement.cpp
index 468883b63cc6996a0e213fcab5ba42a1f867b522..ebcc4553f96ddf07d0c1fdb040eaf71c7cd822c4 100644
--- a/Source/core/dom/PseudoElement.cpp
+++ b/Source/core/dom/PseudoElement.cpp
@@ -81,10 +81,13 @@ void PseudoElement::attach(const AttachContext& context)
     Element::attach(context);
 
     RenderObject* renderer = this->renderer();
-    if (!renderer || !renderer->style()->regionThread().isEmpty())
+    if (!renderer)
         return;
-
     RenderStyle* style = renderer->style();
+    if (!style->regionThread().isEmpty())
+        return;
+    if (style->styleType() != BEFORE && style->styleType() != AFTER)
+        return;
     ASSERT(style->contentData());
 
     for (const ContentData* content = style->contentData(); content; content = content->next()) {