ui/chromeos - Replace stevenjb with khorimoto as owner

Bug: None
Change-Id: If7fb029fa54bad0908695d087e3f5350ca42bf31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2481064Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818050}

ui/chromeos/OWNERS

 afakhry@chromium.org
 jamescook@chromium.org
+khorimoto@chromium.org
 oshima@chromium.org
-stevenjb@chromium.org
 xiyuan@chromium.org
 
 per-file file_manager_strings.grdp=*