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This way we'll get saner and less noisy histogram-based results.

Rationale for moving histogram_utils.py: HistogramValue is part of
telemetry.value, so it makes sense that all utils it needs are there too;
doesn't make sense to import the utils from metrics.

BUG=

Review URL: https://codereview.chromium.org/578593004

Cr-Commit-Position: refs/heads/master@{#295254}