This is followup from the malformed-json bug which took down
Sheriff-o-Matic on 2019-05-16. This CL just adds the canned check;
future CLs can either add it to PanProjectChecks or add it to
other PRESUBMIT files directly.

Change-Id: I4a445193c1744966a448b12c7eb2915873e484d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1617941
Commit-Queue: Aaron Gable <agable@chromium.org>
Reviewed-by: Sean McCullough <seanmccullough@chromium.org>