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

Source/core/dom/Element.cpp

Index: Source/core/dom/Element.cpp
diff --git a/Source/core/dom/Element.cpp b/Source/core/dom/Element.cpp
index 8df5e79ab407fa858be4c1cfe83b049e1602ea14..ba67e72f28ad1ac96924ae89e485ff3bc1dcf49d 100644
--- a/Source/core/dom/Element.cpp
+++ b/Source/core/dom/Element.cpp
@@ -205,6 +205,7 @@ Element::~Element()
         ElementRareData* data = elementRareData();
         data->setPseudoElement(BEFORE, 0);
         data->setPseudoElement(AFTER, 0);
+        data->setPseudoElement(BACKDROP, 0);
         data->clearShadow();
     }
 
@@ -1343,6 +1344,7 @@ void Element::attach(const AttachContext& context)
     ContainerNode::attach(context);
 
     createPseudoElementIfNeeded(AFTER);
+    createPseudoElementIfNeeded(BACKDROP);
 
     if (hasRareData()) {
         ElementRareData* data = elementRareData();
@@ -2402,7 +2404,10 @@ void Element::updatePseudoElement(PseudoId pseudoId, StyleChange change)
 
 void Element::createPseudoElementIfNeeded(PseudoId pseudoId)
 {
-    if (!document()->styleSheetCollection()->usesBeforeAfterRules())
+    if ((pseudoId == BEFORE || pseudoId == AFTER) && !document()->styleSheetCollection()->usesBeforeAfterRules())
+        return;
+
+    if (pseudoId == BACKDROP && !isInTopLayer())
         return;
 
     if (!renderer() || !pseudoElementRendererIsNeeded(renderer()->getCachedPseudoStyle(pseudoId)))