Implement FullScreen using top layer.

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index dff02f7249fa660d7e136ddbdfd24c7ad473be35..3a7ce21466f628bb09e048b38d438655c5e14513 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -5037,9 +5037,13 @@ void Document::removeFromTopLayer(Element* element)
 
 HTMLDialogElement* Document::activeModalDialog() const
 {
-    if (m_topLayerElements.isEmpty())
-        return 0;
-    return toHTMLDialogElement(m_topLayerElements.last().get());
+    for (auto it = m_topLayerElements.rbegin(); it != m_topLayerElements.rend(); ++it) {
+        Element* currentTopLayerElement = it->get();
+        if (isHTMLDialogElement(currentTopLayerElement))
+            return toHTMLDialogElement(currentTopLayerElement);
+    }
+
+    return nullptr;
 }
 
 void Document::exitPointerLock()