Use a direct include of strings headers in chromeos/.

chromeos/disks/mock_disk_mount_manager.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 "chromeos/disks/mock_disk_mount_manager.h"
 
 #include <utility>
 
 #include "base/message_loop.h"
 #include "base/stl_util.h"
-#include "base/string_util.h"
+#include "base/strings/string_util.h"
 
 using testing::_;
 using testing::AnyNumber;
 using testing::Invoke;
 using testing::ReturnRef;
 
 namespace chromeos {
 namespace disks {
 
 namespace {
@@ skipping 215 matching lines @@
   FOR_EACH_OBSERVER(Observer, observers_, OnDiskEvent(event, disk));
 }
 
 void MockDiskMountManager::NotifyDeviceChanged(DeviceEvent event,
                                                const std::string& path) {
   FOR_EACH_OBSERVER(Observer, observers_, OnDeviceEvent(event, path));
 }
 
 }  // namespace disks
 }  // namespace chromeos