Update ListOwners for GerritClient in owners_client

The REST endpoint used by GerritClient.ListOwners needs to be updated as
of https://gerrit-review.googlesource.com/c/plugins/code-owners/+/293806

Change-Id: I5c142f16443b157c9d8edfa5bcf418cbedd2b193
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2658609
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>

owners_client.py

@@ -203,4 +203,4 @@ class GerritClient(OwnersClient):
     # random.
     data = gerrit_util.GetOwnersForFile(
         self._host, self._project, self._branch, path)
-    return [d['account']['email'] for d in data]
+    return [d['account']['email'] for d in data['code_owners']]

tests/owners_client_test.py

@@ -27,7 +27,8 @@ emily = 'emily@example.com'
 
 
 def _get_owners():
-  return [
+  return {
+    "code_owners": [
       {
         "account": {
           "email": 'approver@example.com'
@@ -44,6 +45,7 @@ def _get_owners():
         },
       }
     ]
+  }