Refactor CalculateConfidence and add unit test.

Refactor CalculateConfidence and add unit test.

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=

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=262924

[shatch, 2014-04-08 19:59:06]

Really liking the refactor work! Script is becoming more readable :) The tests
are something that the script has lacked for a while, good to see that started
as well.


lgtm, please let Prasad take a look too.

https://codereview.chromium.org/220113012/diff/20001/tools/bisect-perf-regres...
File tools/bisect-perf-regression_test.py (right):

https://codereview.chromium.org/220113012/diff/20001/tools/bisect-perf-regres...
tools/bisect-perf-regression_test.py:11: class
BisectPerfRegressionTest(unittest.TestCase):
Maybe in the future might be good to add some tests for the bisect script's
output, since tools like the dashboard depend on it.

https://codereview.chromium.org/220113012/diff/20001/tools/bisect-perf-regres...
tools/bisect-perf-regression_test.py:22: def
testCalculateTruncatedMeanRaisesError(self):
Add some tests for truncated mean calculations?

[prasadv, 2014-04-09 23:21:11]

lgtm

[qyearsley, 2014-04-10 00:18:44]

The CQ bit was checked by qyearsley@chromium.org

[commit-bot: I haz the power, 2014-04-10 00:18:57]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/qyearsley@chromium.org/220113012/20001

[commit-bot: I haz the power, 2014-04-10 05:10:19]

Change committed as 262924