Add alcooper@ to OWNERS of xr-related directories

Change-Id: Ie519c2a70970a52b3f8b734c112eed7d2c346245
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610576Reviewed-by: Bill Orr <billorr@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659227}

chrome/browser/vr/service/OWNERS

+alcooper@chromium.org
 billorr@chromium.org
 
 # TEAM: xr-dev@chromium.org

chrome/services/isolated_xr_device/OWNERS

+alcooper@chromium.org
 billorr@chromium.org
 ddorwin@chromium.org
 klausw@chromium.org

device/vr/OWNERS

+alcooper@chromium.org
 bajones@chromium.org
 bialpio@chromium.org
 billorr@chromium.org

third_party/blink/renderer/modules/xr/OWNERS

+alcooper@chromium.org
 bajones@chromium.org
 klausw@chromium.org
 

third_party/webxr_test_pages/OWNERS

+alcooper@chromium.org
 bajones@chromium.org
 bialpio@chromium.org
 billorr@chromium.org