Fix crash in policy template presubmit check

When a boolean policy has a undefined value in the 'items' attributions,
the error message uses undefined variables and causes crash.

Bug: 1165816
Change-Id: I4600d0d8ba51472e8d69035c368f462a7f645b0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2629430
Commit-Queue: Owen Min <zmin@chromium.org>
Commit-Queue: Julian Pastarmov <pastarmovj@chromium.org>
Auto-Submit: Owen Min <zmin@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843643}

components/policy/tools/syntax_check_policy_template_json.py

@@ -633,7 +633,7 @@ class PolicyTemplateChecker(object):
               self._Error(
                   ('Policy %s of type main has an item with a value %s, value '
                    'must be one of %s') %
-                  (policy.get('name'), name, required_names))
+                  (policy.get('name'), value, required_values))
 
       if not values_seen.issuperset(required_values):
         self._Error(