assistant: add ash/public/cpp/assistant/OWNERS

Bug: None
Test: None
Change-Id: Ia0a87528e94620c6952292d04a2f849a07ac07f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1937909Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719656}

ash/public/cpp/assistant/OWNERS

+file://chromeos/assistant/OWNERS
+
+# COMPONENT: UI>Shell>Assistant