Revert of Refactor perf bisect script _CalculateConfidence method. (https://coderevie...

Revert of Refactor perf bisect script _CalculateConfidence method. (https://codereview.chromium.org/209853009/)

Reason for revert:
Caused issue 358622.

The bug was that at line 2832 I used a tuple instead of a list, then later I tried to assign to a member of the tuple, but Python tuples are immutable.

Original issue's description:
> Refactor perf bisect script _CalculateConfidence method.
>
>  - Adding docstrings
>  - Factoring out _CalculateBounds method
>  - Bug fix???
>
> BUG=
>
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=260241

TBR=prasadv@chromium.org,simonhatch@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/218613012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@260885 0039d316-1c4b-4281-b951-d872f2087c98