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

Source/core/rendering/RenderLayer.cpp

Index: Source/core/rendering/RenderLayer.cpp
diff --git a/Source/core/rendering/RenderLayer.cpp b/Source/core/rendering/RenderLayer.cpp
index 568ec4325ca7a17e61fad7d469a296879bd292b0..c0741c2f21f31b7c0d93224bd81ed22daad6753a 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -51,6 +51,7 @@
 #include "core/css/PseudoStyleRequest.h"
 #include "core/dom/Document.h"
 #include "core/dom/DocumentEventQueue.h"
+#include "core/dom/PseudoElement.h"
 #include "core/dom/WebCoreMemoryInstrumentation.h"
 #include "core/dom/shadow/ShadowRoot.h"
 #include "core/editing/FrameSelection.h"
@@ -4265,15 +4266,6 @@ bool RenderLayer::isInTopLayer() const
     return node && node->isElementNode() && toElement(node)->isInTopLayer();
 }
 
-bool RenderLayer::isInTopLayerSubtree() const
-{
-    for (const RenderLayer* layer = this; layer; layer = layer->parent()) {
-        if (layer->isInTopLayer())
-            return true;
-    }
-    return false;
-}
-
 // Compute the z-offset of the point in the transformState.
 // This is effectively projecting a ray normal to the plane of ancestor, finding where that
 // ray intersects target, and computing the z delta between those two points.
@@ -5620,8 +5612,9 @@ void RenderLayer::rebuildZOrderLists(OwnPtr<Vector<RenderLayer*> >& posZOrderLis
         for (RenderObject* child = view->firstChild(); child; child = child->nextSibling()) {
             Element* childElement = (child->node() && child->node()->isElementNode()) ? toElement(child->node()) : 0;
             if (childElement && childElement->isInTopLayer()) {
-                RenderLayer* layer = toRenderLayerModelObject(child)->layer();
-                posZOrderList->append(layer);
+                if (Element* backdrop = childElement->pseudoElement(BACKDROP))
+                    posZOrderList->append(toRenderLayerModelObject(backdrop->renderer())->layer());
+                posZOrderList->append(toRenderLayerModelObject(child)->layer());
             }
         }
     }
@@ -5648,7 +5641,7 @@ void RenderLayer::updateNormalFlowList()
 
 void RenderLayer::collectLayers(bool includeHiddenLayers, OwnPtr<Vector<RenderLayer*> >& posBuffer, OwnPtr<Vector<RenderLayer*> >& negBuffer, const RenderLayer* layerToForceAsStackingContainer, CollectLayersBehavior collectLayersBehavior)
 {
-    if (isInTopLayer())
+    if (isInTopLayer() || isBackdrop())
         return;
 
     updateDescendantDependentFlags();