Introduce a public WebNotificationPermission enumeration.

We use WebNotificationPresenter::Permission in many places in Chromium,
both in the renderer and browser processes, so it should have its own file.

Also make the WebNotificationPermissionCallback destructor public since
the embedder will take be taking ownership of the instances.

BUG=392187

Review URL: https://codereview.chromium.org/376303002

git-svn-id: svn://svn.chromium.org/blink/trunk@177865 bbb929c8-8fbe-4397-9dbb-9b2b20218538