The function TestingJsonParser::Start uses base::ThreadTaskRunnerHandle to post tasks to the current thread. This breaks when the code runs in a task scheduler worker thread (see the use sequences not threads in the task scheduler documentation).

https://cs.chromium.org/chromium/src/components/safe_json/testing_json_parser.cc?rcl=56456075157c17d59a70ee3f938c40c35f9bec96&l=47

Ideally, we want to change this to use SequencedTaskRunnerHandle::Get() instead of ThreadTaskRunnerHandle::Get().

The code in question is called during unit tests by the supervised user whitelist installer.