Fix Python3 usage with perf tests/gold tests. Bug: angleproject:6733 Change-Id: I9d1baf78ed182db77c6a6d8e896737413ca417f3 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3304012 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>