[Notifications] Non-persistent notifications now use Mojo path

- Switched non-persistent notifications over to the mojo path and
removed the legacy IPC message handlers and surrounding code.

R=peter@chromium.org

Bug: 796990
Change-Id: I49baccc562ffacb5f76f4cc3067d60824b269daf
Reviewed-on: https://chromium-review.googlesource.com/908448
Reviewed-by: Emily Stark <estark@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Anita Woodruff <awdf@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536071}
17 files changed