Add owners for web_ax_object.h, web_ax_enums.h

Add Blink a11y team to OWNERS for web_ax_object.h
Add aleventhal to OWNERS for web_ax_enums.h

Bug: None
Change-Id: Ibebb187146e177bae68302731367026db9d6400a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2623190Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842528}

third_party/blink/public/web/OWNERS

-per-file web_ax_enums.h=dmazzoni@chromium.org
-per-file web_ax_enums.h=aboxhall@chromium.org
+# Accessibility
+per-file web_ax_object.h=file://third_party/blink/renderer/modules/accessibility/OWNERS
+per-file web_ax_enums.h=file://third_party/blink/renderer/modules/accessibility/OWNERS
 per-file web_ax_enums.h=dtseng@chromium.org
-per-file web_ax_enums.h=nektar@chromium.org
 
 # Page load metrics
 per-file web_performance.h=bmcquade@chromium.org