MessageCenterView.

- Update login state notification blocker to hide all notifications in
  message center when screen is locked to maintain the behavior:
      "Don't make notifications read when opening the message 
       center on lock screen"
- Remove methods/variables/structs related to UnreadCount, which is
  not used anywhere except in tests.
- It's questionable whether we have any reason to still track is_read
  at all, but it's used for UMA in at least one place.

Bug: 755413
Change-Id: Ic00a24900992735870338a9544e6af61da744974
Reviewed-on: https://chromium-review.googlesource.com/757132


Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515765}