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

Source/core/dom/ContainerNode.cpp

Index: Source/core/dom/ContainerNode.cpp
diff --git a/Source/core/dom/ContainerNode.cpp b/Source/core/dom/ContainerNode.cpp
index 5fdfa0c736a25f75642599581f2e6fcf84cf2eaa..19d79ab8704453a72d111d5a649e1d7f66f26f27 100644
--- a/Source/core/dom/ContainerNode.cpp
+++ b/Source/core/dom/ContainerNode.cpp
@@ -707,7 +707,7 @@ void ContainerNode::notifyNodeRemoved(Node& root)
         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);