Add khmel@ to three chrome/browser/chromeos/arc/*/OWNERS files

BUG=b:116761000
TEST=None

Change-Id: I353b0b183df8242b9a799face70be560f186e56c
Reviewed-on: https://chromium-review.googlesource.com/1252386Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Yury Khmel <khmel@chromium.org>
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595624}

chrome/browser/chromeos/arc/app_shortcuts/OWNERS

+khmel@chromium.org

chrome/browser/chromeos/arc/extensions/OWNERS

+khmel@chromium.org
+hidehiko@chromium.org

chrome/browser/chromeos/arc/optin/OWNERS

+khmel@chromium.org
+hidehiko@chromium.org