Revert of Check to ensure PowerObservers are added and removed on the same thread

base/power_monitor/power_observer.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 BASE_POWER_MONITOR_POWER_OBSERVER_H_
 #define BASE_POWER_MONITOR_POWER_OBSERVER_H_
 
 #include "base/base_export.h"
 #include "base/compiler_specific.h"
-#include "base/memory/scoped_ptr.h"
-#include "base/threading/thread_checker.h"
 
 namespace base {
 
 class BASE_EXPORT PowerObserver {
-  friend class PowerMonitor;
-
  public:
-  PowerObserver();
-
   // Notification of a change in power status of the computer, such
   // as from switching between battery and A/C power.
-  virtual void OnPowerStateChange(bool on_battery_power) {}
+  virtual void OnPowerStateChange(bool on_battery_power) {};
 
   // Notification that the system is suspending.
   virtual void OnSuspend() {}
 
   // Notification that the system is resuming.
   virtual void OnResume() {}
 
  protected:
-  virtual ~PowerObserver();
-
- private:
-  // Allows PowerMonitor to ensure this observer is added and removed
-  // from the same thread, which is required by the ObserverListThreadSafe.
-  scoped_ptr<base::ThreadChecker> power_monitor_thread_checker_;
+  virtual ~PowerObserver() {}
 };
 
 }  // namespace base
 
 #endif  // BASE_POWER_MONITOR_POWER_OBSERVER_H_