REGRESSION(r219857): run-benchmark --allplans broken

REGRESSION(r219857): run-benchmark --allplans broken
https://bugs.webkit.org/show_bug.cgi?id=175186

Reviewed by Saam Barati.

r219857 forgot to update also the calls to BenchmarkRunner() that
is done when the script is run with --allplans.

To fix this (and avoid future issues like this), let's factorize
the calls to the benchhmark runner in a run_benchmark_plan()
function.

* Scripts/webkitpy/benchmark_runner/run_benchmark.py:
(run_benchmark_plan):
(start):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@220292 268f45cc-cd09-0410-ab3c-d52691b4dbfc