Add a UseCounter for Notification.permission

While this is a relatively new property, it does require a synchronous IPC
to the browser process in order to get the permission status, and thus would
be better as a Notification.hasPermission() method returning a Promise (or
similar).

Since we will need a similar construct for Push Messaging, and potentially
other APIs, let's get some data on whether developers use this to see if
we can get a more common, asynchronous approach.

BUG=

Review URL: https://codereview.chromium.org/254023002

git-svn-id: svn://svn.chromium.org/blink/trunk@173004 bbb929c8-8fbe-4397-9dbb-9b2b20218538