Today, Web Notifications get a random Id assigned to them by the delegate
used with the message_center::Notification object. This has a number of
implications:

  (1) We don't get super smooth "update" transitions for message center
      notifications upon replacing a notification.
  (2) We continue to rely on the delegates associated with Notification
      objects, which in themselves are incompatible with notifications
      that can outlive the process they were created by.
  (3) It's not possible for the //content layer, that has knowledge of the
      notifications that *should* be showing, to close them when certain
      events occur (for instance, the Service Worker unregistering). That
      leads to unresponsive notifications.

This patch makes the NotificationIdGenerator responsible for creating the
notification Ids, giving us a clear path towards solving these issues. In
addition, it further isolates the concept of persistent notification Ids,
reducing the number of Ids readers of this code have to keep in mind.

The NotificationDatabase's keying scheme has been changed to refer to the
generated Id as opposed to the ever-incrementing persistent notification
ID. This avoids needing separate association keys and removes a DELETE
operation prior to writing data for a potentially replaced notification.

TBR=mkwst (type change in /web/)
BUG=485985, 439950

Review-Url: https://codereview.chromium.org/2300093002
Cr-Commit-Position: refs/heads/master@{#418241}