Does this require triage or is it clear that this will be done / who will do it?

Also, is there value in checking this in 2. if 1. is always run at compile time anyway?

I'll take it.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7aaf9dbf524c43452769ffe58f2a436de1741946

commit 7aaf9dbf524c43452769ffe58f2a436de1741946
Author: Alexander Hendrich <hendrich@chromium.org>
Date: Mon Aug 13 09:03:00 2018

Check that properties in 'required' are in the 'properties' object.

Verifiy that all 'required' properties in a policy schema are actually
existing in 'properties' as well.

Bug:  856903 
Test: components_unittests --gtest_filter=JSONSchemaValidator.*
Change-Id: Ie8c3ff8ac82bcb3c436cfb580721070050b66b22
Reviewed-on: https://chromium-review.googlesource.com/1165361
Reviewed-by: Lutz Justen <ljusten@chromium.org>
Commit-Queue: Alexander Hendrich <hendrich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582533}
[modify] https://crrev.com/7aaf9dbf524c43452769ffe58f2a436de1741946/components/json_schema/json_schema_validator.cc
[modify] https://crrev.com/7aaf9dbf524c43452769ffe58f2a436de1741946/components/json_schema/json_schema_validator_unittest.cc
[modify] https://crrev.com/7aaf9dbf524c43452769ffe58f2a436de1741946/components/policy/tools/generate_policy_source.py

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bf75bad5d18f8190981830d28e8eec257d300e43

commit bf75bad5d18f8190981830d28e8eec257d300e43
Author: Alexander Hendrich <hendrich@chromium.org>
Date: Wed Aug 22 14:02:20 2018

Add python schema validator

This CL adds a python schema validator that can validate JSON schemas.
The schema validator is used in a presubmit to validate all the schemas
in policy_templates.json. The schema validator itself supports the
minimal set of features needed for our current policies.

Bug:  873641 ,  856903 ,  856901 
Change-Id: Ie577e825807e4438eca6921722b88af7c8d1781c
Reviewed-on: https://chromium-review.googlesource.com/1172433
Commit-Queue: Alexander Hendrich <hendrich@chromium.org>
Reviewed-by: Lutz Justen <ljusten@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585003}
[add] https://crrev.com/bf75bad5d18f8190981830d28e8eec257d300e43/components/policy/tools/schema_validator.py
[modify] https://crrev.com/bf75bad5d18f8190981830d28e8eec257d300e43/components/policy/tools/syntax_check_policy_template_json.py

Please provide steps if this requires manual verification. Thanks.! 

hendrich@ Could you please help us with repro steps to verify the fix on #70.0.3534.0

Sorry, this doesn't need manual verfication as it is a presubmit check only.