This showed up here and made the bot red: https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac10.13%20Tests%20%28dbg%29/7444

Looking at the flakiness dashboard, this test times out often across platforms: https://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=NavigatingExtensionPopupBrowserTest.DownloadViaPost

Can we make the test more reliable?