Fix typo in perf_test_runner.py. Bug: angleproject:2923 Change-Id: I2e961e18373cc1c317700f2f6b64a0b9bf32e4a2 Reviewed-on: https://chromium-review.googlesource.com/c/1340200 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>