content: Cleanup class/struct forward declarations

content/browser/renderer_host/media/media_devices_manager.h

 // Copyright 2016 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 CONTENT_BROWSER_RENDERER_HOST_MEDIA_MEDIA_DEVICES_MANAGER_H_
 #define CONTENT_BROWSER_RENDERER_HOST_MEDIA_MEDIA_DEVICES_MANAGER_H_
 
 #include <array>
 #include <vector>
 
 #include "base/callback.h"
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "base/system_monitor/system_monitor.h"
 #include "content/common/content_export.h"
 #include "content/common/media/media_devices.h"
 #include "media/capture/video/video_capture_device_descriptor.h"
 
-namespace base {
-class SingleThreadTaskRunner;
-}
-
 namespace media {
 class AudioManager;
 }
 
 namespace content {
 
 class MediaStreamManager;
 class VideoCaptureManager;
 
 // Use MediaDeviceType values to index on this type.
@@ skipping 117 matching lines @@
   bool monitoring_started_;
 
   base::WeakPtrFactory<MediaDevicesManager> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(MediaDevicesManager);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_RENDERER_HOST_MEDIA_MEDIA_DEVICES_MANAGER_H_