Add xiaohuic to wallpaper manager owners

Bug: None
Change-Id: If207621ff75593d95028c02d89cb9a95d5777e7a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2615105
Auto-Submit: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841125}

chrome/browser/resources/chromeos/wallpaper_manager/OWNERS

 xdai@chromium.org
+xiaohuic@chromium.org