Rename notification_id() to disambiguate its meaning

ui/message_center/notification.h

Index: ui/message_center/notification.h
diff --git a/ui/message_center/notification.h b/ui/message_center/notification.h
index e151f4318259770dfcd2c6d4cb80af362455d607..7a58363e75d8ab0b9298db88be3beeeb287237ae 100644
--- a/ui/message_center/notification.h
+++ b/ui/message_center/notification.h
@@ -66,8 +66,6 @@ class MESSAGE_CENTER_EXPORT Notification {
 
   Notification(const Notification& other);
 
-  Notification& operator=(const Notification& other);
-
   virtual ~Notification();
 
   // Copies the internal on-memory state from |base|, i.e. shown_as_popup,
@@ -202,6 +200,8 @@ class MESSAGE_CENTER_EXPORT Notification {
       const base::Closure& click_callback);
 
  protected:
+  Notification& operator=(const Notification& other);
+
   // The type of notification we'd like displayed.
   NotificationType type_;