De-flake HostResolverImplTest.IsIPv6Reachable.

net/dns/host_resolver_impl.cc

Index: net/dns/host_resolver_impl.cc
diff --git a/net/dns/host_resolver_impl.cc b/net/dns/host_resolver_impl.cc
index e4ee073591d764edfd0f20e786493da4eef2b7dd..60fd4742b8e572742a4c176f5fd423a23caa9c18 100644
--- a/net/dns/host_resolver_impl.cc
+++ b/net/dns/host_resolver_impl.cc
@@ -2336,12 +2336,14 @@ HostResolverImpl::Key HostResolverImpl::GetEffectiveKeyForRequest(
 }
 
 bool HostResolverImpl::IsIPv6Reachable(const NetLogWithSource& net_log) {
-  base::TimeTicks now = base::TimeTicks::Now();
+  // Cache the result for kIPv6ProbePeriodMs (measured from after
+  // IsGloballyReachable() completes).
   bool cached = true;
-  if ((now - last_ipv6_probe_time_).InMilliseconds() > kIPv6ProbePeriodMs) {
+  if ((base::TimeTicks::Now() - last_ipv6_probe_time_).InMilliseconds() >
+      kIPv6ProbePeriodMs) {
     last_ipv6_probe_result_ =
         IsGloballyReachable(IPAddress(kIPv6ProbeAddress), net_log);
-    last_ipv6_probe_time_ = now;
+    last_ipv6_probe_time_ = base::TimeTicks::Now();
     cached = false;
   }
   net_log.AddEvent(NetLogEventType::HOST_RESOLVER_IMPL_IPV6_REACHABILITY_CHECK,