[Mac][MC] Implement notification updates and relayouts.

ui/message_center/cocoa/notification_controller.h

Index: ui/message_center/cocoa/notification_controller.h
diff --git a/ui/message_center/cocoa/notification_controller.h b/ui/message_center/cocoa/notification_controller.h
index 8768e778a1930e09f94fcdd8e85948db1dde1a49..5f68856396846bddd97c16c4ff884f27e46ec3e9 100644
--- a/ui/message_center/cocoa/notification_controller.h
+++ b/ui/message_center/cocoa/notification_controller.h
@@ -51,6 +51,10 @@ MESSAGE_CENTER_EXPORT
 - (id)initWithNotification:(const message_center::Notification*)notification
     messageCenter:(message_center::MessageCenter*)messageCenter;
 
+// If the model object changes, this method will update the views to reflect
+// the new model object. Returns the updated frame of the notification.
+- (NSRect)updateNotification:(const message_center::Notification*)notification;
+
 // Action for clicking on the notification's |closeButton_|.
 - (void)close:(id)sender;