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As the Hash dance UMA histograms will be used for testing, it's
preferred to use their constants rather than literal strings.

This CL is exposing all Hash dance UMA histograms to policy
namespace.

BUG=chromium:1157317

Change-Id: Idf72d0d66cf0d2585d77e2b811f5ec0fc78a4041
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2582339
Commit-Queue: Amr Aboelkher <amraboelkher@google.com>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835811}