Make Event RefCountedGarbageCollected and implement trace() methods to the Event hierarcy

Source/modules/device_orientation/DeviceMotionEvent.h

Index: Source/modules/device_orientation/DeviceMotionEvent.h
diff --git a/Source/modules/device_orientation/DeviceMotionEvent.h b/Source/modules/device_orientation/DeviceMotionEvent.h
index 6a638cae7da83a48e804594f946ac39a814bb785..ce19d5fb6403af6c04cac2b439e34a45fff75700 100644
--- a/Source/modules/device_orientation/DeviceMotionEvent.h
+++ b/Source/modules/device_orientation/DeviceMotionEvent.h
@@ -58,15 +58,16 @@ public:
 
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     DeviceMotionEvent();
     DeviceMotionEvent(const AtomicString& eventType, DeviceMotionData*);
 
-    RefPtrWillBePersistent<DeviceMotionData> m_deviceMotionData;
-
-    RefPtrWillBePersistent<DeviceAcceleration> m_acceleration;
-    RefPtrWillBePersistent<DeviceAcceleration> m_accelerationIncludingGravity;
-    RefPtrWillBePersistent<DeviceRotationRate> m_rotationRate;
+    RefPtrWillBeMember<DeviceMotionData> m_deviceMotionData;
+    RefPtrWillBeMember<DeviceAcceleration> m_acceleration;
+    RefPtrWillBeMember<DeviceAcceleration> m_accelerationIncludingGravity;
+    RefPtrWillBeMember<DeviceRotationRate> m_rotationRate;
 };
 
 DEFINE_TYPE_CASTS(DeviceMotionEvent, Event, event, event->interfaceName() == EventNames::DeviceMotionEvent, event.interfaceName() == EventNames::DeviceMotionEvent);