See FindIt results:
https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVy6AELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCKxAWNocm9taXVtLm1hYy9NYWMxMC4xMSBUZXN0cy8xMTUyNS9icm93c2VyX3NpZGVfbmF2aWdhdGlvbl9icm93c2VyX3Rlc3RzIG9uIE1hYy0xMC4xMS9VMEpPWVhacFoyRjBhVzl1VDJKelpYSjJaWEpDY205M2MyVnlWR1Z6ZEM1RWFYSmxZM1JFYjNkdWJHOWhaRTV2VW1WbVpYSnlaWEpVWVhKblpYUkNiR0Z1YXc9PQwLEhNNYXN0ZXJGbGFrZUFuYWx5c2lzGAEM

Example failure:
https://luci-logdog.appspot.com/v/?s=chromium%2Fbb%2Ftryserver.chromium.mac%2Fmac_chromium_variable_deflake%2F242%2F%2B%2Frecipes%2Fsteps%2Ftest_95f01e9%2F0%2Fsteps%2Fbrowser_side_navigation_browser_tests__95f01e9__on_Mac-10.11%2F0%2Flogs%2FSBNavigationObserverBrowserTest.DirectDownloadNoReferrerTargetBlank%2F0


../../chrome/browser/safe_browsing/safe_browsing_navigation_observer_browsertest.cc:228: Failure
Value of: download_items.size()
  Actual: 0
Expected: 1U
Which is: 1
BrowserTestBase received signal: Segmentation fault: 11. Backtrace:
0   browser_tests                       0x000000010351d3cc base::debug::StackTrace::StackTrace(unsigned long) + 28
1   browser_tests                       0x0000000103c64668 content::(anonymous namespace)::DumpStackTraceSignalHandler(int) + 200
2   libsystem_platform.dylib            0x00007fff9982252a _sigtramp + 26
3   ???                                 0x0000000000000000 0x0 + 0
4   browser_tests                       0x0000000101cdd136 safe_browsing::SBNavigationObserverBrowserTest_DirectDownloadNoReferrerTargetBlank_Test::RunTestOnMainThread() + 902
5   browser_tests                       0x0000000103c643b7 content::BrowserTestBase::ProxyRunTestOnMainThreadLoop() + 327
6   browser_tests                       0x0000000103651ef6 ChromeBrowserMainParts::PreMainMessageLoopRunImpl() + 4358


It looks like we open a new tab with target = _blank which resolves to a download (signed.exe). The CL to service the CreateNewWindow IPC on the UI thread [1] triggered flakiness here, but I'm still not sure of the root cause.

[1]: https://codereview.chromium.org/2821473002/