Make sure the rootNode of a LiveNodeListBase is always a ContainerNode

Source/web/WebNode.cpp

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 26 matching lines @@
 #include "WebDOMEventListener.h"
 #include "WebDocument.h"
 #include "WebElement.h"
 #include "WebFrameImpl.h"
 #include "WebNodeList.h"
 #include "WebPluginContainer.h"
 #include "WebPluginContainerImpl.h"
 #include "bindings/v8/ExceptionState.h"
 #include "core/dom/Document.h"
 #include "core/dom/Element.h"
+#include "core/dom/EmptyNodeList.h"
 #include "core/events/Event.h"
 #include "core/dom/Node.h"
 #include "core/dom/NodeList.h"
 #include "core/editing/markup.h"
 #include "core/rendering/RenderObject.h"
 #include "core/rendering/RenderWidget.h"
 #include "platform/Widget.h"
 #include "public/platform/WebString.h"
 #include "public/platform/WebVector.h"
 
@@ skipping 128 matching lines @@
     return false;
 }
 
 void WebNode::simulateClick()
 {
     m_private->dispatchSimulatedClick(0);
 }
 
 WebNodeList WebNode::getElementsByTagName(const WebString& tag) const
 {
-    return WebNodeList(m_private->getElementsByTagName(tag));
+    if (m_private->isContainerNode())
+        return WebNodeList(toContainerNode(m_private.get())->getElementsByTagName(tag));
+    return WebNodeList(EmptyNodeList::create(m_private.get()));
 }
 
 WebElement WebNode::querySelector(const WebString& tag, WebExceptionCode& ec) const
 {
     TrackExceptionState exceptionState;
     WebElement element(m_private->querySelector(tag, exceptionState));
     ec = exceptionState.code();
     return element;
 }
 
@@ skipping 54 matching lines @@
     m_private = node;
     return *this;
 }
 
 WebNode::operator PassRefPtr<Node>() const
 {
     return m_private.get();
 }
 
 } // namespace blink