Revert of Attempting to resolve a race condition with PowerMonitor

base/power_monitor/power_monitor_source.h

Index: base/power_monitor/power_monitor_source.h
diff --git a/base/power_monitor/power_monitor_source.h b/base/power_monitor/power_monitor_source.h
index 883eddd4388aa1d3232fd496a2412181f1badb09..b8f41850ef2502e70f84c4a2d82e411f0ef289e2 100644
--- a/base/power_monitor/power_monitor_source.h
+++ b/base/power_monitor/power_monitor_source.h
@@ -40,6 +40,8 @@
   // 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