Add OWNERS file for webrtc_cases/.

Bug: None
Change-Id: I619491887eb97abc493debfeaf20ae260c222f33
Reviewed-on: https://chromium-review.googlesource.com/568820Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486147}

tools/perf/page_sets/OWNERS

+per-file update_webrtc_cases=ehmaldonado@chromium.org
+per-file update_webrtc_cases=jansson@chromium.org

tools/perf/page_sets/webrtc_cases/OWNERS

+ehmaldonado@chromium.org
+jansson@chromium.org