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This CL introduces a core logic class to pick scheduled notifications
to show.

Basically it checks the configuration and impression history, and
iterate all types of notification to return a list of notification.

Also implements a unit test frame work for this class, more test cases
will be added in following CLs.

Bug: 930968
Change-Id: Ia35519fe7a8509c53fa84ab380d42214c59d4e75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1546672
Commit-Queue: Xing Liu <xingliu@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646648}