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This CL sets up the unit test for notification scheduler glue layer.

Also initialization is covered in this CL, other logic will be covered
in following CLs.

Bug: 963304
Change-Id: If2ae52428a7262a8eac23e7ab2efdd2d6bbbb1dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1730601Reviewed-by: Hesen Zhang <hesen@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684052}