Do not use virtual function to request shutdown

Would make it easier to port to other platforms.

atom/browser/api/atom_api_power_monitor.h

@@ -26,16 +26,20 @@ class PowerMonitor : public mate::TrackableObject<PowerMonitor>,
   explicit PowerMonitor(v8::Isolate* isolate);
   ~PowerMonitor() override;
 
+  // Called by native calles.
+  bool ShouldShutdown();
+
+#if defined(OS_LINUX)
+  // Private JS APIs.
+  void BlockShutdown();
+  void UnblockShutdown();
+#endif
+
   // base::PowerObserver implementations:
   void OnPowerStateChange(bool on_battery_power) override;
   void OnSuspend() override;
   void OnResume() override;
 
-#if defined(OS_LINUX)
-  // atom::PowerObserver
-  bool OnShutdown() override;
-#endif
-
  private:
   DISALLOW_COPY_AND_ASSIGN(PowerMonitor);
 };