chrome: Cleanup class/struct forward declarations

chrome/browser/data_use_measurement/chrome_data_use_ascriber_service.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 CHROME_BROWSER_DATA_USE_MEASUREMENT_CHROME_DATA_USE_ASCRIBER_SERVICE_H_
 #define CHROME_BROWSER_DATA_USE_MEASUREMENT_CHROME_DATA_USE_ASCRIBER_SERVICE_H_
 
 #include <list>
 
 #include "base/macros.h"
 #include "components/keyed_service/core/keyed_service.h"
 
-class IOThread;
-
 namespace content {
 class NavigationHandle;
 class RenderFrameHost;
 }
 
 namespace data_use_measurement {
 class ChromeDataUseAscriber;
 
 // UI thread functionality of ChromeDataUseAscriber.
 //
@@ skipping 53 matching lines @@
   // is set. The RenderFrameHost pointers in the queues are valid for the
   // duration that they are in the queue.
   std::list<content::RenderFrameHost*> pending_frames_queue_;
 
   DISALLOW_COPY_AND_ASSIGN(ChromeDataUseAscriberService);
 };
 
 }  // namespace data_use_measurement
 
 #endif  // CHROME_BROWSER_DATA_USE_MEASUREMENT_CHROME_DATA_USE_ASCRIBER_SERVICE_H_