This makes the test robust for stress GC flags.

Bug: 
Change-Id: Ica65987f0ee09fbdb4aab233dea4c51db5b19459
Reviewed-on: https://chromium-review.googlesource.com/702436Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48313}