Add accessibility team to third_party/closure_compiler/externs OWNERS file

Changes to these externs are mechanical and already enforced by a presubmit
check, so it doesn't make sense to get owners review most of the time. We'll
use our best judgement and loop in an owner if the change is nontrivial.

NOTRY=true

Bug: none
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I8a4ab89ae7cf231cffcf313b631b3c878b693b31
Reviewed-on: https://chromium-review.googlesource.com/1062075
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559178}

third_party/closure_compiler/externs/OWNERS

 michaelpg@chromium.org
 rdevlin.cronin@chromium.org
 stevenjb@chromium.org
+
+per-file accessibility_private.js=file://ui/accessibility/OWNERS
+per-file automation.js=file://ui/accessibility/OWNERS