Oilpan: tidy up reference to the Document's "CSS target".

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 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 689 matching lines @@
     NoEventDispatchAssertion assertNoEventDispatch;
 
     Document& document = root.document();
     for (Node* node = &root; node; node = NodeTraversal::next(*node, &root)) {
         // As an optimization we skip notifying Text nodes and other leaf nodes
         // of removal when they're not in the Document tree since the virtual
         // call to removedFrom is not needed.
         if (!node->inDocument() && !node->isContainerNode())
             continue;
         if (document.cssTarget() == node)
-            document.setCSSTarget(0);
+            document.setCSSTarget(nullptr);
         node->removedFrom(this);
         for (ShadowRoot* shadowRoot = node->youngestShadowRoot(); shadowRoot; shadowRoot = shadowRoot->olderShadowRoot())
             notifyNodeRemoved(*shadowRoot);
     }
 }
 
 void ContainerNode::attach(const AttachContext& context)
 {
     attachChildren(context);
     clearChildNeedsStyleRecalc();
@@ skipping 557 matching lines @@
         return true;
 
     if (node->isElementNode() && toElement(node)->shadow())
         return true;
 
     return false;
 }
 #endif
 
 } // namespace WebCore