ash: Cleanup class/struct forward declarations

ash/metrics/desktop_task_switch_metric_recorder.h

 // Copyright 2015 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 ASH_METRICS_DESKTOP_TASK_SWITCH_METRIC_RECORDER_H_
 #define ASH_METRICS_DESKTOP_TASK_SWITCH_METRIC_RECORDER_H_
 
 #include "ash/ash_export.h"
 #include "base/macros.h"
 #include "ui/wm/public/activation_change_observer.h"
 
 namespace ash {
 
-class UserMetricsRecorder;
-
 // Tracks metrics for task switches caused by the user activating a task window
 // by clicking or tapping on it.
 class ASH_EXPORT DesktopTaskSwitchMetricRecorder
     : public aura::client::ActivationChangeObserver {
  public:
   explicit DesktopTaskSwitchMetricRecorder();
   ~DesktopTaskSwitchMetricRecorder() override;
 
   // aura::client::ActivationChangeObserver:
   void OnWindowActivated(
       aura::client::ActivationChangeObserver::ActivationReason reason,
       aura::Window* gained_active,
       aura::Window* lost_active) override;
 
  private:
   // Tracks the last active task window.
   aura::Window* last_active_task_window_;
 
   DISALLOW_COPY_AND_ASSIGN(DesktopTaskSwitchMetricRecorder);
 };
 
 }  // namespace ash
 
 #endif  // ASH_METRICS_DESKTOP_TASK_SWITCH_METRIC_RECORDER_H_