presubmit: Fix CheckOwnersFormat

Fix CheckOwnersFormat so that it checks the new OWNERS files instead of the
original ones.

Change-Id: I4abb1cfc847bd28f45b9948fe7775c10dcc8a420
Reviewed-on: https://chromium-review.googlesource.com/c/1382946
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

presubmit_canned_checks.py

@@ -840,7 +840,9 @@ def CheckOwnersFormat(input_api, output_api):
   if not affected_files:
     return []
   try:
-    input_api.owners_db.load_data_needed_for(affected_files)
+    owners_db = input_api.owners_db
+    owners_db.override_files = {}
+    owners_db.load_data_needed_for(affected_files)
     return []
   except Exception as e:
     return [output_api.PresubmitError(