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This CL moves the PlatformNotificationContext and the
NotificationDatabaseData classes into the //content API. Serialization
behavior for NotificationDatabaseData will remain content-private.

The primary users of this will be (1) clearing browser data, (2) UA-
triggered notification closures and (3) forced push notifications.

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

BUG=447628

Review URL: https://codereview.chromium.org/1010833002

Cr-Commit-Position: refs/heads/master@{#320993}