tl;dr:   The test flakes whenever crypto::EnsureNSSInit() takes longer than 1 second. (Which on my system is happening semi-reliably)

This happens through an awkward set of dependencies:

   HostResolverImpl::IsIPv6Reachable()
   IsGloballyReachable()
   ClientSocketFactory::GetDefaultFactory()
   CertDatabase::GetInstance()
   CertDatabase()
   NSSInitSingleton()
   NSS_InitReadWrite  <=== SLOW

The test is weird in how it depends on the time it takes to run IsGloballyReachable(). I can easily fix that part to de-flake it.

However this will also need follow-up from issue 455942, which is addressing the general jankiness of IsGloballyReachable(). This might be one contributor (or the primary one) to the slowness seen in that bug thread. But it would only contribute to jank on the very first call to IsGloballyReachable(), and only on platforms using NSS.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1a17ef38f0fd22d8c297524072b4530cb1a2664a

commit 1a17ef38f0fd22d8c297524072b4530cb1a2664a
Author: eroman <eroman@chromium.org>
Date: Wed Dec 14 01:06:09 2016

De-flake HostResolverImplTest.IsIPv6Reachable.

Previously the test would fail if IsGloballyReachable() took longer
than 1 second.

(The test still has a dependency on some actions executing within a 1
second window, but unlikely to hit it).

BUG= 673059 

Review-Url: https://codereview.chromium.org/2562073003
Cr-Commit-Position: refs/heads/master@{#438373}

[modify] https://crrev.com/1a17ef38f0fd22d8c297524072b4530cb1a2664a/net/dns/host_resolver_impl.cc