Support slot element's fallback content feature

third_party/WebKit/Source/core/dom/ContainerNode.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2013 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 1515 matching lines @@
 
 #if ENABLE(ASSERT)
 bool childAttachedAllowedWhenAttachingChildren(ContainerNode* node)
 {
     if (node->isShadowRoot())
         return true;
 
     if (node->isInsertionPoint())
         return true;
 
+    if (isHTMLSlotElement(node))
+        return true;
+
     if (node->isElementNode() && toElement(node)->shadow())
         return true;
 
     return false;
 }
 #endif
 
 } // namespace blink