Add per-file rule for BUILD.gn to ui/arc/OWNERS

Bug: None
Change-Id: Id96dc2fb2cfc84ef1512690c24c1f000ef0cb01c
Reviewed-on: https://chromium-review.googlesource.com/966002Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543647}

ui/arc/OWNERS

+# This is for the common case of adding or renaming files. If you're doing
+# structural changes, use usual OWNERS rules.
+per-file BUILD.gn=*
+
 elijahtaylor@chromium.org
 hidehiko@chromium.org
 lhchavez@chromium.org