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This class provides an interface, to be used on the IO thread, on top
of the NotificationDatabase class. It abstracts away the thread
requirements of the database class and is responsible for managing the
status and the lifetime of the database.

Right now it's only used in unit tests, but it will eventually live as
a member of StoragePartitionImpl, and will be used by the notification
message filter.

Design document:
  http://goo.gl/TciXVp

BUG=447628

Review URL: https://codereview.chromium.org/1006493005

Cr-Commit-Position: refs/heads/master@{#320781}