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- Enables the mojo pathway by default and removes the legacy IPC
code path for persistent notifications.

- Removes the NotificationsWithMojo feature flag which previously
guarded this code-path during incremental development.

R=kinuko@chromium.org

Bug: 796991
Change-Id: Ia4dac0ab4b98b7a6df342b7df1b424e3af67bf05
Reviewed-on: https://chromium-review.googlesource.com/1027843Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Anita Woodruff <awdf@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557486}