Fire null Device Motion events only once.

Source/modules/device_orientation/DeviceSensorEventController.h

Index: Source/modules/device_orientation/DeviceSensorEventController.h
diff --git a/Source/modules/device_orientation/DeviceSensorEventController.h b/Source/modules/device_orientation/DeviceSensorEventController.h
index 816ac9b86e417c76d4e0b9b4318e0f58df2ec926..9fdce9aec687ab4c1833f1857b37c3fa4b8b895a 100644
--- a/Source/modules/device_orientation/DeviceSensorEventController.h
+++ b/Source/modules/device_orientation/DeviceSensorEventController.h
@@ -49,12 +49,14 @@ protected:
     virtual PassRefPtr<Event> getLastEvent() = 0;
     virtual void registerWithDispatcher() = 0;
     virtual void unregisterWithDispatcher() = 0;
+    virtual bool isNullEvent(Event*) = 0;
 
 private:
     void fireDeviceEvent(Timer<DeviceSensorEventController>*);
 
     Document* m_document;
     bool m_isActive;
+    bool m_needsCheckingNullEvents;
     Timer<DeviceSensorEventController> m_timer;
 };