Treat unknown fields as errors

Change-Id: Ia3beb34686f34f4e712fc682a88a843967070656
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2458406Reviewed-by: Owen Min <zmin@chromium.org>
Commit-Queue: Chris Sharp <csharp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814851}

components/policy/tools/syntax_check_policy_template_json.py

@@ -513,8 +513,7 @@ class PolicyTemplateChecker(object):
           'arc_support',
           'supported_chrome_os_management',
       ):
-        self._Warning('In policy %s: Warning: Unknown key: %s' %
-                      (policy.get('name'), key))
+        self._Error('In policy %s: Unknown key: %s' % (policy.get('name'), key))
 
     # Each policy must have a name.
     self._CheckContains(policy, 'name', str, regexp_check=NO_WHITESPACE)