Add estaab to .gn OWNERS for 'gn check' code health effort.

Change-Id: I806dab8cea22aa3cd429b3c54727a4c5cb66fc4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2288493Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Erik Staab <estaab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786518}

OWNERS

@@ -10,6 +10,7 @@ per-file .gitattributes=*
 per-file .gitignore=*
 per-file .git-blame-ignore-revs=mgiuca@chromium.org
 per-file .git-blame-ignore-revs=thakis@chromium.org
+per-file .gn=estaab@chromium.org
 per-file .gn=file://build/OWNERS
 per-file .vpython*=dpranke@chromium.org
 per-file .vpython*=dpranke@google.com