Use nullptr instead of 0 in xml and xmlhttprequest.

Source/core/xml/XPathNodeSet.cpp

Index: Source/core/xml/XPathNodeSet.cpp
diff --git a/Source/core/xml/XPathNodeSet.cpp b/Source/core/xml/XPathNodeSet.cpp
index 7cba9d039099ec46e736d11b01a8cb2b0f972f3b..a8d96afde270866859a175d856f9f7a4ec04ee2e 100644
--- a/Source/core/xml/XPathNodeSet.cpp
+++ b/Source/core/xml/XPathNodeSet.cpp
@@ -263,7 +263,7 @@ void NodeSet::reverse()
 Node* NodeSet::firstNode() const
 {
     if (isEmpty())
-        return 0;
+        return nullptr;
 
     // FIXME: fully sorting the node-set just to find its first node is
     // wasteful.
@@ -274,7 +274,7 @@ Node* NodeSet::firstNode() const
 Node* NodeSet::anyNode() const
 {
     if (isEmpty())
-        return 0;
+        return nullptr;
 
     return m_nodes.at(0).get();
 }