Make perf_test_runner Python 2.7 and 3 compatible. BUG=None Change-Id: I915c99f21dc879a8e63955cd6a3dd892d05bbf5f Reviewed-on: https://chromium-review.googlesource.com/646590 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>