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Specifically, this adds tests for:
  TruncatedMean
  Variance
  StandardDeviation
  StandardError

which were not covered before. I also added a couple notes about things
we may want to change in the behavior of the math_utils functions.

Still not covered is PooledStandardError, unit test coverage can be added
for that function in separate CL.

BUG=

Review URL: https://codereview.chromium.org/736573002

Cr-Commit-Position: refs/heads/master@{#305052}