Add branch-day SA as owner of the supported_branches.json file

Bug: b:486796365
Change-Id: I11a14692bc8014786cce4da8ae1b0e8718aca73e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1182740
Reviewed-by: Eric Boren <borenet@google.com>

OWNERS

diff --git a/OWNERS b/OWNERS
index f2c2ddf..fc4b68c 100644
--- a/OWNERS
+++ b/OWNERS
@@ -5,4 +5,5 @@
 kjlubick@google.com
 lovisolo@google.com
 rmistry@google.com
-sharaks@google.com
\ No newline at end of file
+sharaks@google.com
+per-file supported-branches.json=chrome-branch-day@chops-service-accounts.iam.gserviceaccount.com
\ No newline at end of file