The notification database should be able to read or delete multiple items.

The notification database should be able to read or delete multiple items.

This adds five methods to the NotificationDatabase class to read or delete
multiple notifications, which share two actual implementations.

They will be used as follows:
- Read all data: browsing data overviews.
- Read all data for an origin: for origin storage overviews, quota
  client integration (storageQuota.queryInfo()).
- Read all data for a SWR id: to get all notifications associated
  with a given SWR (SWR.getNotifications()).
- Delete all data for an origin: clearing browsing data for a
  given origin.
- Delete all data for a SWR id: to remove notifications when a service
  worker registration gets dropped.

Actual integration with these features will be done in separate patches,
this patch just introduces the primitives in the database.

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

BUG=447628

Review URL: https://codereview.chromium.org/1019073002

Cr-Commit-Position: refs/heads/master@{#321399}