Add utf_string_conversions to base namespace.

chrome/browser/extensions/api/serial/serial_port_enumerator_win.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/extensions/api/serial/serial_port_enumerator.h"
 
 #include <windows.h>
 
 #include "base/memory/scoped_ptr.h"
 #include "base/win/registry.h"
 #include "base/string_util.h"
 #include "base/stringprintf.h"
 
 namespace extensions {
 
 SerialPortEnumerator::StringSet
 SerialPortEnumerator::GenerateValidSerialPortNames() {
   StringSet name_set;
 
   base::win::RegistryValueIterator iter_key(HKEY_LOCAL_MACHINE,
       L"HARDWARE\\DEVICEMAP\\SERIALCOMM\\");
 
   for (; iter_key.Valid(); ++iter_key) {
     string16 str(iter_key.Value());
-    std::string device_string(WideToASCII(str));
+    std::string device_string(base::WideToASCII(str));
     name_set.insert(device_string);
   }
   return name_set;
 }
 
 }  // namespace extensions