Add owners to //ash/accessibility

Bug: none
Change-Id: I22c12174321b8471e54bfb65c9062fe367671471
Reviewed-on: https://chromium-review.googlesource.com/827628Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524219}

ash/accessibility/OWNERS

+file://ui/accessibility/OWNERS
+
+# TEAM: chromium-accessibility@chromium.org
+# COMPONENT: UI>Accessibility