NotificationController::clientFrom() should return a reference

NotificationController has maintained NotificationClient with OwnPtr since r173468. And, NotificationClient should not be a null. We can return a reference on NotificationController::clientFrom().

BUG=none

Review URL: https://codereview.chromium.org/268353004

git-svn-id: svn://svn.chromium.org/blink/trunk@175795 bbb929c8-8fbe-4397-9dbb-9b2b20218538