[Notification] Call finalize() when stop() is called not to leak Notification object

Otherwise, Notification object leaks when navigation or reload happens. See content::NotificationProvider::OnNavigate().

BUG=none

Review URL: https://codereview.chromium.org/30943004

git-svn-id: svn://svn.chromium.org/blink/trunk@161784 bbb929c8-8fbe-4397-9dbb-9b2b20218538