Add Peter as an owner of the JSTests benchmark

This information is useful to know whom to assign bugs to when these tests are
crashing on our infrastructure.

R=petermarshall@chromium.org

No-Try: true
Change-Id: Ia165e0236602cae73e144011537d642e3535fa6b
Reviewed-on: https://chromium-review.googlesource.com/908563
Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51259}

test/js-perf-test/OWNERS

+per-file JSTests.json=petermarshall@chromium.org