Consistently use AtomicString for namespaceURI / prefix

Source/core/xml/NativeXPathNSResolver.h

Index: Source/core/xml/NativeXPathNSResolver.h
diff --git a/Source/core/xml/NativeXPathNSResolver.h b/Source/core/xml/NativeXPathNSResolver.h
index 17a1238ba10d79d700940e1abd5ded5c40c1675a..79523100b4d205c94af6603a1331cd941e38651d 100644
--- a/Source/core/xml/NativeXPathNSResolver.h
+++ b/Source/core/xml/NativeXPathNSResolver.h
@@ -38,7 +38,7 @@ public:
     static PassRefPtr<NativeXPathNSResolver> create(PassRefPtr<Node> node) { return adoptRef(new NativeXPathNSResolver(node)); }
     virtual ~NativeXPathNSResolver();
 
-    virtual String lookupNamespaceURI(const String& prefix);
+    virtual AtomicString lookupNamespaceURI(const String& prefix) OVERRIDE;
 
 private:
     explicit NativeXPathNSResolver(PassRefPtr<Node>);