cppgc: Add owners for cppgc samples

Bug: chromium:1056170
Change-Id: I3779ed504235e3aa3fbecb5f14dd2d43d9907c07
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2560201Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71420}

samples/cppgc/OWNERS

+bikineev@chromium.org
+mlippautz@chromium.org
+omerkatz@chromium.org