The test was using add_result_callback() which is triggered after a test
finishes. Unfortunately this file uses both async_test and promise_test.
promise_tests run sequentially but async_tests run in parallel. Therefore,
the add_result_callback was being called every time an async_test finished,
causing a race when this promise_test was in the middle of execution.

R=shimazu

Bug: 807014
Change-Id: Ic1bc568fe812f540feec901f95cc89684ef1cae2
Reviewed-on: https://chromium-review.googlesource.com/892553
Reviewed-by: Makoto Shimazu <[email protected]>
Commit-Queue: Matt Falkenhagen <[email protected]>
Cr-Commit-Position: refs/heads/master@{#532758}