Add core/aom/OWNERS

It just includes modules/accessibility/OWNERS.

No-Try: True
Change-Id: Ia3ab082249993c9d2789b4668ebca8c0a08fd81c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1611058
Commit-Queue: Alice Boxhall <aboxhall@chromium.org>
Auto-Submit: Kent Tamura <tkent@chromium.org>
Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659442}

third_party/blink/renderer/core/aom/OWNERS

+file://third_party/blink/renderer/modules/accessibility/OWNERS
+
+# TEAM: chromium-accessibility@chromium.org
+# COMPONENT: Blink>Accessibility