Allow COMMON_OWNERS to LGTM .mailmap CLs

Bug: chromium:1213438
Change-Id: I52e381841b888e0f5edc0bad9e0319e840829914
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2982600
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Auto-Submit: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75346}

OWNERS

@@ -10,6 +10,7 @@ per-file .git-blame-ignore-revs=file:INFRA_OWNERS
 per-file .gitattributes=file:INFRA_OWNERS
 per-file .gitignore=file:INFRA_OWNERS
 per-file .gn=file:INFRA_OWNERS
+per-file .mailmap=file:COMMON_OWNERS
 per-file .vpython=file:INFRA_OWNERS
 per-file .ycm_extra_conf.py=file:INFRA_OWNERS
 per-file .bazelrc=file:COMMON_OWNERS