Fix JSBench which causes math domain error in run-benchmark script.

Fix JSBench which causes math domain error in run-benchmark script.
https://bugs.webkit.org/show_bug.cgi?id=157697
<rdar://problem/26279292>

Reviewed by Ryosuke Niwa.

Some tests of JSBench give time usage to be 0 ms which will causes error while calculating geometric mean.
Also fix a round-off error while calculating stdev.

* Scripts/webkitpy/benchmark_runner/benchmark_results.py:
(BenchmarkResults._format_values):
* Scripts/webkitpy/benchmark_runner/data/patches/JSBench.patch:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@200914 268f45cc-cd09-0410-ab3c-d52691b4dbfc