Use nullptr instead of 0 in xml and xmlhttprequest.

Source/core/xml/XSLStyleSheetLibxslt.cpp

Index: Source/core/xml/XSLStyleSheetLibxslt.cpp
diff --git a/Source/core/xml/XSLStyleSheetLibxslt.cpp b/Source/core/xml/XSLStyleSheetLibxslt.cpp
index 75c6483404ee9c2547d3eaea5efc739cc277522f..70fc6a07a32500c151b008f9b1e08b40a2f83257 100644
--- a/Source/core/xml/XSLStyleSheetLibxslt.cpp
+++ b/Source/core/xml/XSLStyleSheetLibxslt.cpp
@@ -269,7 +269,7 @@ Document* XSLStyleSheet::ownerDocument()
         if (node)
             return &node->document();
     }
-    return 0;
+    return nullptr;
 }
 
 xmlDocPtr XSLStyleSheet::locateStylesheetSubResource(xmlDocPtr parentDoc, const xmlChar* uri)