Add --additional-drt-flag option to run-perf-tests to make it easy to test runtime options
https://bugs.webkit.org/show_bug.cgi?id=111021

Reviewed by Dirk Pranke.

The underlying code (which is shared with run-webkit-tests)
already knew how to support this option, it just wasn't exposed
via the run-perf-tests front-end.  This patch fixes that.

* Scripts/webkitpy/performance_tests/perftestsrunner.py:
(PerfTestsRunner._parse_args):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144253 268f45cc-cd09-0410-ab3c-d52691b4dbfc