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Summary of changes:
 * Piped demographic metrics provider with chrome browser (iOS + others).
 * Expose user demographics from DemographicMetricsProvider.

TBR=treib@chromium.org

Bug: 979371
Change-Id: Ib0d1eff9b767cf121fe768a9a46a3ff1fb57dd27
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1687019
Commit-Queue: Vincent Boisselle <vincb@google.com>
Reviewed-by: Roger McFarlane <rogerm@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680540}