Use a direct include of strings headers, delete some strings forwarding headers.

chrome/browser/ui/cocoa/notifications/message_center_tray_bridge_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 "chrome/browser/ui/cocoa/notifications/message_center_tray_bridge.h"
 
 #include "base/memory/scoped_nsobject.h"
 #include "base/message_loop.h"
 #include "base/run_loop.h"
-#include "base/utf_string_conversions.h"
+#include "base/strings/utf_string_conversions.h"
 #import "ui/message_center/cocoa/status_item_view.h"
 #include "ui/message_center/message_center.h"
 #include "ui/message_center/notification.h"
 #import "ui/base/test/ui_cocoa_test_helper.h"
 
 class MessageCenterTrayBridgeTest : public ui::CocoaTest {
  public:
   virtual void SetUp() OVERRIDE {
     ui::CocoaTest::SetUp();
 
@@ skipping 39 matching lines @@
 
   EXPECT_TRUE(status_item());
   EXPECT_EQ(1u, [status_item() unreadCount]);
 
   center_->RemoveNotification("1", /*by_user=*/true);
 
   base::RunLoop().RunUntilIdle();
 
   EXPECT_FALSE(status_item());
 }