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This CL adds glue interfaces and setup unimplemented classes for
notification scheduler system to use platform background task scheduler
API.

TBR=peter@chromium.org

Bug: 930968
Change-Id: I01abc4edc15f4d2569ec7fb0abf012341cfa4ee5
Reviewed-on: https://chromium-review.googlesource.com/c/1470718Reviewed-by: Xing Liu <xingliu@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636185}