- Remove Notification::tag_ (for web notification types, where this is relevant,
  the tag, if provided, is cooked into the ID)
- Replace notifications based on ID (along with profile). For most notifications,
  this should be a no-op. For those that previously didn't specify tags, this may
  mean less notification spam as two notifications with the same ID will not
  be shown together.
- Remove c/b/notifications/Notification and just use message_center::Notification.

Bug: 763426
Change-Id: Id9ef84a40c51b28f27b951ca27e357c53408b998
Reviewed-on: https://chromium-review.googlesource.com/699797


Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509437}