Gracefully handle a disconnected popup menu client on disposing open popup.

Source/core/dom/Node.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, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 901 matching lines @@
     return detachingNode == this;
 }
 #endif
 
 void Node::detach(const AttachContext& context)
 {
     ASSERT(document().lifecycle().stateAllowsDetach());
     DocumentLifecycle::DetachScope willDetach(document().lifecycle());
 
 #if ENABLE(ASSERT)
-    ASSERT(!detachingNode);
+    // The detaching might trigger destruction of a popup menu window,
+    // with ensuing detachment of its Nodes. In a separate document, so
+    // don't assert for these, but do set detachingNode to the most recent
+    // Node being detached.
+    ASSERT(!detachingNode || detachingNode->document() != document());
     detachingNode = this;
 #endif
 
     if (renderer())
         renderer()->destroyAndCleanupAnonymousWrappers();
     setRenderer(nullptr);
 
     // Do not remove the element's hovered and active status
     // if performing a reattach.
     if (!context.performingReattach) {
@@ skipping 1501 matching lines @@
         node->showTreeForThis();
 }
 
 void showNodePath(const blink::Node* node)
 {
     if (node)
         node->showNodePathForThis();
 }
 
 #endif