Intermittent failure in /_mozilla/mozilla/task_queue_throttling.any.html

[jdm]

{
    "status": "FAIL", 
    "group": "default", 
    "message": "assert_true: expected true got false", 
    "stack": "perf_observer@http://web-platform.test:8000/_mozilla/mozilla/task_queue_throttling.any.js:11:5\nTest.prototype.step@http://web-platform.test:8000/resources/testharness.js:1566:20\nTest.prototype.step_func_done/<@http://web-platform.test:8000/resources/testharness.js:1606:17\n", 
    "subtest": "Throttling the performance timeline task queue.", 
    "test": "/_mozilla/mozilla/task_queue_throttling.any.html", 
    "line": 173699, 
    "action": "test_result", 
    "expected": "PASS"
}

[gterzian]

This looks like the same as #21552

Problem I think is that when the results of the file reading in parallel are "slow" to be enqueued as tasks, we don't get the throttling since the tasks haven't been enqueued yet, and the performance timeline task ends-up being run first...

The solution would be somehow ensure, perhaps by using something else than a file reader, that tasks meant to 'fill-up' the queue are indeed enqueued by the next iteration of the event-loop(and not later).

The solution should preferably work both in workers and browsing-contexts event-loops...