Revert of Attempting to resolve a race condition with PowerMonitor

base/power_monitor/power_monitor_source.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_MONITOR_SOURCE_H_
 #define BASE_POWER_MONITOR_POWER_MONITOR_SOURCE_H_
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/memory/ref_counted.h"
@@ skipping 22 matching lines @@
 
  protected:
   friend class PowerMonitorTest;
 
   // Friend function that is allowed to access the protected ProcessPowerEvent.
   friend void ProcessPowerEventHelper(PowerEvent);
 
   // Get the process-wide PowerMonitorSource (if not present, returns NULL).
   static PowerMonitorSource* Get();
 
+  // ProcessPowerEvent should only be called from a single thread, most likely
+  // the UI thread or, in child processes, the IO thread.
   static void ProcessPowerEvent(PowerEvent event_id);
 
   // Platform-specific method to check whether the system is currently
   // running on battery power.  Returns true if running on batteries,
   // false otherwise.
   virtual bool IsOnBatteryPowerImpl() = 0;
 
  private:
   bool on_battery_power_;
   bool suspended_;
 
   // This lock guards access to on_battery_power_, to ensure that
   // IsOnBatteryPower can be called from any thread.
   Lock battery_lock_;
 
   DISALLOW_COPY_AND_ASSIGN(PowerMonitorSource);
 };
 
 }  // namespace base
 
 #endif  // BASE_POWER_MONITOR_POWER_MONITOR_SOURCE_H_