NotificationController handles an ownership of NotificaitonClient

NotificationClient is only used by NotificationController. So, NotificationController is able to have ownership of the client by using OwnPtr. The client's life cycle is more clear via clear ownership definition.

BUG=none

Review URL: https://codereview.chromium.org/263883004

git-svn-id: svn://svn.chromium.org/blink/trunk@173468 bbb929c8-8fbe-4397-9dbb-9b2b20218538