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This is a reland of 34d462f6

Original change's description:
> Add end-to-end tests for demographic metrics in UMA and UKM
> 
> Bug: 992573
> Change-Id: I5a27682828538c72fbe8fc71c78cbadc8fdef23e
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872643
> Commit-Queue: Roger McFarlane <rogerm@chromium.org>
> Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
> Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#728621}

Bug: 992573
Change-Id: Ib6cf80256a8adbcb15a23814f6431d7ea600a1b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1994003Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Vincent Boisselle <vincb@google.com>
Cr-Commit-Position: refs/heads/master@{#730183}