Update Assistant opt-in UI owners

Change-Id: I15fe2194703605918de6a3253d978e7e14ccae31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2046172
Commit-Queue: Denis Kuznetsov [CET] <antrim@chromium.org>
Auto-Submit: Denis Kuznetsov [CET] <antrim@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740276}

chrome/browser/resources/chromeos/assistant_optin/OWNERS

+updowndota@chromium.org
+
+#This UI is a part of OOBE/First login:
+file://chrome/browser/resources/chromeos/login/OWNERS