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

chrome/browser/notifications/notification_audio_controller.h

 // 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.
 
 #ifndef CHROME_BROWSER_NOTIFICATIONS_NOTIFICATION_AUDIO_CONTROLLER_H_
 #define CHROME_BROWSER_NOTIFICATIONS_NOTIFICATION_AUDIO_CONTROLLER_H_
 
 #include <string>
 
 #include "base/callback.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_vector.h"
-#include "base/string_piece.h"
+#include "base/strings/string_piece.h"
 #include "media/audio/audio_parameters.h"
 
 namespace base {
 class MessageLoopProxy;
 }
 
 class Profile;
 
 // This class controls the sound playing for notifications. Note that this class
 // belongs to the audio thread and self-owned.
@@ skipping 45 matching lines @@
 
   scoped_refptr<base::MessageLoopProxy> message_loop_;
   ScopedVector<AudioHandler> audio_handlers_;
 
   media::AudioParameters::Format output_format_;
 
   DISALLOW_COPY_AND_ASSIGN(NotificationAudioController);
 };
 
 #endif  // CHROME_BROWSER_NOTIFICATIONS_NOTIFICATION_AUDIO_CONTROLLER_H_