Add mrefaat@ as owner for cookie changes in web/browsingdata

I own most of Cookies related code in the codebase, and i created
system_cookie_store_util which was moved in this directory.

Change-Id: Ib025b980c111f528cd0180a16a82909a70eec305
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2382875
Commit-Queue: Mohammad Refaat <mrefaat@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803173}

ios/web/browsing_data/OWNERS

 eugenebut@chromium.org
 gambard@chromium.org
 
+per-file system_cookie_store_util_unittest.mm=mrefaat@chromium.org
+per-file system_cookie_store_util.mm=mrefaat@chromium.org
+
+
 # TEAM: ios-directory-owners@chromium.org
 # OS: iOS
 # COMPONENT: Mobile>iOSWeb