Allow benchmarks to provide the standard deviation.

Some benchmarks include their own runner which provides an
overall average and a standard deviation. This enables
extraction of that value similar to the other measurements.

These benchmarks should only be run once. If a benchmarks
specifies multiple runs and provides a standard deviation,
a warning will be issued that makes the build fail on the
buildbot side.

TEST=python -m unittest run_benchmarks_test
BUG=393947
LOG=n
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/395633012

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22424 ce2b1a6d-e550-0410-aec6-3dcde31c8c00