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Reporting is a spec for delivering out-of-band reports from various
other parts of the browser. See http://wicg.github.io/reporting/ for
the spec, or https://goo.gl/pygX5I for details of the planned
implementation in Chromium.

This adds an Observer interface by which parts of Reporting can observe
cache updates. Expected users are the DeliveryAgent and Serializer.

BUG=704259

Review-Url: https://codereview.chromium.org/2778373003
Cr-Commit-Position: refs/heads/master@{#462980}