Fire null Device Motion events only once.

Source/modules/device_orientation/DeviceMotionController.cpp

Index: Source/modules/device_orientation/DeviceMotionController.cpp
diff --git a/Source/modules/device_orientation/DeviceMotionController.cpp b/Source/modules/device_orientation/DeviceMotionController.cpp
index b001f486f7a6c5c6c7c62bcc7c4384221a598eee..4e28bf2f254c1fc02e9604ff9f4edb99c56ad7e5 100644
--- a/Source/modules/device_orientation/DeviceMotionController.cpp
+++ b/Source/modules/device_orientation/DeviceMotionController.cpp
@@ -28,6 +28,7 @@
 #include "modules/device_orientation/DeviceMotionController.h"
 
 #include "core/dom/Document.h"
+#include "modules/device_orientation/DeviceMotionData.h"
 #include "modules/device_orientation/DeviceMotionDispatcher.h"
 #include "modules/device_orientation/DeviceMotionEvent.h"
 
@@ -82,4 +83,11 @@ void DeviceMotionController::unregisterWithDispatcher()
     DeviceMotionDispatcher::instance().removeDeviceMotionController(this);
 }
 
+bool DeviceMotionController::isNullEvent(Event* event)
+{
+    ASSERT(event->type() == eventNames().devicemotionEvent);
+    DeviceMotionEvent* motionEvent = static_cast<DeviceMotionEvent*>(event);
+    return !motionEvent->deviceMotionData()->canProvideEventData();
+}
+
 } // namespace WebCore