Add OWNERS to src/protobuf

R=petermarshall@chromium.org

Bug: v8:9247
Change-Id: Ia78904788bf7523b1ec5aa0494c79d71ee121030
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1609847Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61517}

src/protobuf/OWNERS

+petermarshall@chromium.org