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 655974630dee69d467fa9cd954fcae6341c410de..66fb5b92a82a6eba7ffdaee9ce5fd05651191c2d 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();
     }
 
@@ -1292,7 +1293,10 @@ void Element::removedFrom(ContainerNode* insertionPoint)
     if (Element* after = pseudoElement(AFTER))
         after->removedFrom(insertionPoint);
 
+    if (Element* backdrop = pseudoElement(BACKDROP))
+        backdrop->removedFrom(insertionPoint);
     document()->removeFromTopLayer(this);
+
     if (containsFullScreenElement())
         setContainsFullScreenElementOnAncestorsCrossingFrameBoundaries(false);
 
@@ -1350,6 +1354,7 @@ void Element::attach(const AttachContext& context)
     ContainerNode::attach(context);
 
     createPseudoElementIfNeeded(AFTER);
+    createPseudoElementIfNeeded(BACKDROP);
 
     if (hasRareData()) {
         ElementRareData* data = elementRareData();
@@ -1382,6 +1387,7 @@ void Element::detach(const AttachContext& context)
         ElementRareData* data = elementRareData();
         data->setPseudoElement(BEFORE, 0);
         data->setPseudoElement(AFTER, 0);
+        data->setPseudoElement(BACKDROP, 0);
         data->setIsInCanvasSubtree(false);
         data->resetComputedStyle();
         data->resetDynamicRestyleObservations();
@@ -2418,7 +2424,10 @@ 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)))
@@ -2429,7 +2438,10 @@ void Element::createPseudoElementIfNeeded(PseudoId pseudoId)
 
     ASSERT(!isPseudoElement());
     RefPtr<PseudoElement> element = PseudoElement::create(this, pseudoId);
+    if (pseudoId == BACKDROP)
+        document()->addToTopLayer(element.get(), this);
     element->attach();
+
     ensureElementRareData()->setPseudoElement(pseudoId, element.release());
 }