Adding gayane@ and tmartino@ to SH component OWNERS

Per discussion on team chat

Change-Id: Ibb2747aadabce3b12204a02d4e4d8f39be0af5d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2635182
Commit-Queue: Tommy Martino <tmartino@chromium.org>
Commit-Queue: sebsg <sebsg@chromium.org>
Auto-Submit: Tommy Martino <tmartino@chromium.org>
Reviewed-by: sebsg <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844581}

components/shared_highlighting/OWNERS

+gayane@chromium.org
 sebsg@chromium.org
 seblalancette@chromium.org
+tmartino@chromium.org