GitLab

- Remove unused function NotifierSource::GetNotifierType
- Rename NotifierSource to NotifierController
- Add docs

Bug: 755413
Change-Id: Ib214b452548c348661569fe4496a06e67004809e
Reviewed-on: https://chromium-review.googlesource.com/682108Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505266}