Remove NOTIFICATION_BACKGROUND_CONTENTS_*

Replace with a new observer interface and by extending
BackgroundService::Delegate.

Bug: 268984
Change-Id: I3952ae91064a171f88574987a0cef7bacccd663c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1659595
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670198}
17 files changed