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This follows the very same pattern as the read and write methods, but does
so for deleting notifications instead. Note that the notification database
still isn't hooked up to production code, but is covered by a broad
series of unit tests.

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

BUG=447628

Review URL: https://codereview.chromium.org/1008003003

Cr-Commit-Position: refs/heads/master@{#320914}