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This CL adds the notification client interface, and a registrar class
to register and maintain ownership of clients.

The registrar is plumbed into the scheduler keyed service. The internal
code will hook to the client in a following CL.

Bug: 963288,930968
Change-Id: I48eb8cf460260304ab1ebd6de201a4ffc224ddc0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1616013
Commit-Queue: Xing Liu <xingliu@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664089}