This patch enables perf_test_runner.py to run on Linux and MacOS.

BUG=angleproject:2029

Change-Id: Ia7176a3f5fed0245ab72d5def1199623c4fb4fe2
Reviewed-on: https://chromium-review.googlesource.com/497648Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>