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

Source/core/css/PseudoStyleRequest.h

Index: Source/core/css/PseudoStyleRequest.h
diff --git a/Source/core/css/PseudoStyleRequest.h b/Source/core/css/PseudoStyleRequest.h
index 97ff599bf6548e4658a24a96a5d54ae4bd3707ac..fcd0cea32565eabaf237c462a927c4b1bd660a41 100644
--- a/Source/core/css/PseudoStyleRequest.h
+++ b/Source/core/css/PseudoStyleRequest.h
@@ -38,6 +38,12 @@ public:
     {
     }
 
+    // The spec disallows inheritance for ::backdrop.
+    bool allowsInheritance(const RenderStyle* parentStyle) const
+    {
+        return parentStyle && pseudoId != BACKDROP;
+    }
+
     PseudoId pseudoId;
     ScrollbarPart scrollbarPart;
     RenderScrollbar* scrollbar;