Add charles as an owner of assist_ranker.

Change-Id: I2bef2e12d3a5fa5ac42535da709bfe9d337fdaa1
Reviewed-on: https://chromium-review.googlesource.com/892181Reviewed-by: Philippe Hamel <hamelphi@chromium.org>
Reviewed-by: Roger McFarlane <rogerm@chromium.org>
Commit-Queue: Roger McFarlane <rogerm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532983}

components/assist_ranker/OWNERS

+charleszhao@chromium.org
 hamelphi@chromium.org
-rogerm@chromium.org
+rogerm@chromium.org
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