Reduce frequency of IPv6 probes in HostResolverImpl.

net/dns/host_resolver_impl.h

Index: net/dns/host_resolver_impl.h
diff --git a/net/dns/host_resolver_impl.h b/net/dns/host_resolver_impl.h
index 80907424645ca4b43b7255de554eb0e2e7eef05b..e6a7a6db73f23f34874e41bddd78db126880a040 100644
--- a/net/dns/host_resolver_impl.h
+++ b/net/dns/host_resolver_impl.h
@@ -200,7 +200,12 @@ class NET_EXPORT HostResolverImpl
   // family when the request leaves it unspecified.
   Key GetEffectiveKeyForRequest(const RequestInfo& info,
                                 const IPAddressNumber* ip_number,
-                                const BoundNetLog& net_log) const;
+                                const BoundNetLog& net_log);
+
+  // Probes IPv6 support and returns true if IPv6 support is enabled.
+  // Results are cached, i.e. when called repeatedly this method returns result
+  // from the first probe for some time before probing again.
+  bool IsIPv6Reachable(const BoundNetLog& net_log);
 
   // Records the result in cache if cache is present.
   void CacheResult(const Key& key,
@@ -284,6 +289,9 @@ class NET_EXPORT HostResolverImpl
   // local IPv6 connectivity. Disables probing.
   bool use_local_ipv6_;
 
+  base::TimeTicks last_ipv6_probe_time_;
+  bool last_ipv6_probe_result_;
+
   // True iff ProcTask has successfully resolved a hostname known to have IPv6
   // addresses using ADDRESS_FAMILY_UNSPECIFIED. Reset on IP address change.
   bool resolved_known_ipv6_hostname_;