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

base/power_monitor/power_observer.h

Index: base/power_monitor/power_observer.h
diff --git a/base/power_monitor/power_observer.h b/base/power_monitor/power_observer.h
index e7d40a3a6d5a3eb07e1bb45632d236eea3858d21..6be70bba9d5368dfa0cf7a4f145247a1334430ec 100644
--- a/base/power_monitor/power_observer.h
+++ b/base/power_monitor/power_observer.h
@@ -7,20 +7,14 @@
 
 #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() {}
@@ -29,12 +23,7 @@
   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