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This CL implements the ability to get notifications for a given Service
Worker up to the point where it hits the browser process, where there's
a NOTIMPLEMENTED() waiting for now. The browser process then replies
with an empty vector of open notifications.

The second part of the implementation depends on the Notification
Database, which is still an in-progress work.

This CL is part of a three-sided patch:
  [1] https://codereview.chromium.org/1018613003/
  [2] This CL.
  [3] https://codereview.chromium.org/1016843002/

BUG=442143

Review URL: https://codereview.chromium.org/1014073002

Cr-Commit-Position: refs/heads/master@{#321564}