Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/modules/notifications/DOMWindowNotifications.cpp

Index: Source/modules/notifications/DOMWindowNotifications.cpp
diff --git a/Source/modules/notifications/DOMWindowNotifications.cpp b/Source/modules/notifications/DOMWindowNotifications.cpp
index 689d36f40e965216dbc60e29bd7791dd2abba214..65790d96982b9c78aa148156f514a9a64e1e09db 100644
--- a/Source/modules/notifications/DOMWindowNotifications.cpp
+++ b/Source/modules/notifications/DOMWindowNotifications.cpp
@@ -69,13 +69,13 @@ NotificationCenter* DOMWindowNotifications::webkitNotifications(DOMWindow& windo
 
 void DOMWindowNotifications::willDestroyGlobalObjectInFrame()
 {
-    m_notificationCenter = 0;
+    m_notificationCenter = nullptr;
     DOMWindowProperty::willDestroyGlobalObjectInFrame();
 }
 
 void DOMWindowNotifications::willDetachGlobalObjectFromFrame()
 {
-    m_notificationCenter = 0;
+    m_notificationCenter = nullptr;
     DOMWindowProperty::willDetachGlobalObjectFromFrame();
 }