Add xiyuan to //c/b/ui/ash/OWNERS

He's already in //ash/OWNERS and this code is mostly delegates/clients
of the ash code.

Bug: none
Change-Id: I9e11ebc25c8aab35073760f7548cceb5325b5303
Reviewed-on: https://chromium-review.googlesource.com/966542Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543854}

chrome/browser/ui/ash/OWNERS

@@ -3,8 +3,8 @@ jamescook@chromium.org
 oshima@chromium.org
 sky@chromium.org
 stevenjb@chromium.org
+xiyuan@chromium.org
 
-per-file session*=xiyuan@chromium.org
 per-file chrome_keyboard_ui*=yhanada@chromium.org
 per-file keyboard_*=yhanada@chromium.org
 per-file *wallpaper*=file://ash/wallpaper/OWNERS