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This version is a little bit different from the last; main differences:

 - I saw that the "CalculateBounds" was returning the minimum and maximum of the means of the input list, so I thought it would be easier to understand if this was done by mapping CalculateMean and then directly making a tuple with min/max.
 - Since CalculateTruncatedMean(xs, 0) is just the mean, and this is used in several place, I extracted out a convenience function CalculateMean.
 - CalculateConfidence doesn't refer to self, so I thought it was reasonable to move it up to be a top-level function with the other stats functions.

BUG=

Review URL: https://codereview.chromium.org/220113012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@262924 0039d316-1c4b-4281-b951-d872f2087c98