Update the remaining references to sys_string_conversions.h to its new location.

ui/message_center/cocoa/popup_controller_unittest.mm

 // Copyright (c) 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.
 
 #import "ui/message_center/cocoa/popup_controller.h"
 
 #include "base/memory/scoped_nsobject.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/strings/sys_string_conversions.h"
 #include "base/utf_string_conversions.h"
-#include "base/sys_string_conversions.h"
 #import "ui/base/test/ui_cocoa_test_helper.h"
 #include "ui/message_center/notification.h"
 #include "ui/message_center/notification_change_observer.h"
 
 class PopupControllerTest : public ui::CocoaTest {
 };
 
 TEST_F(PopupControllerTest, Creation) {
   scoped_ptr<message_center::Notification> notification(
       new message_center::Notification(
           message_center::NOTIFICATION_TYPE_SIMPLE,
           "",
           ASCIIToUTF16("Added to circles"),
           ASCIIToUTF16("Jonathan and 5 others"),
           string16(),
           std::string(),
           NULL));
 
   scoped_nsobject<MCPopupController> controller(
       [[MCPopupController alloc] initWithNotification:notification.get()
                                        changeObserver:NULL]);
 
   EXPECT_TRUE([controller window]);
   EXPECT_EQ(notification.get(), [controller notification]);
 
   [controller showWindow:nil];
   [controller close];
 }