Use COMMON_OWNERS for top-level BUILD.gn

R=machenbach@chromium.org

Change-Id: I2682e2321513c372940181041e66444388cd1515
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1648263
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62048}

OWNERS

@@ -11,7 +11,7 @@ per-file .gitignore=file://INFRA_OWNERS
 per-file .gn=file://INFRA_OWNERS
 per-file .vpython=file://INFRA_OWNERS
 per-file .ycm_extra_conf.py=file://INFRA_OWNERS
-per-file BUILD.gn=file://INFRA_OWNERS
+per-file BUILD.gn=file://COMMON_OWNERS
 per-file *DEPS=file://INFRA_OWNERS
 per-file PRESUBMIT=file://INFRA_OWNERS
 per-file codereview.settings=file://INFRA_OWNERS