Add base:: to string16 in extensions/.

extensions/common/permissions/permission_message.cc

 // Copyright 2013 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 "extensions/common/permissions/permission_message.h"
 
 namespace extensions {
 //
 // PermissionMessage
 //
 
 PermissionMessage::PermissionMessage(
-    PermissionMessage::ID id, const string16& message)
+    PermissionMessage::ID id, const base::string16& message)
   : id_(id),
     message_(message) {
 }
 
 PermissionMessage::PermissionMessage(
-    PermissionMessage::ID id, const string16& message, const string16& details)
+    PermissionMessage::ID id,
+    const base::string16& message,
+    const base::string16& details)
   : id_(id),
     message_(message),
     details_(details) {
 }
 
 PermissionMessage::~PermissionMessage() {}
 
 }  // namespace extensions