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Implement it for Android, Desktop, and iOS.

This largely supersedes chrome://popular-sites-internals/. Relative to
that, it lacks a way to see the cached JSON and the status of a manual
fetch.

BUG=655622

Review-Url: https://codereview.chromium.org/2557103004
Cr-Commit-Position: refs/heads/master@{#438205}