[net/dns] Instrument DnsConfigService to measure performance and failures.

net/dns/dns_config_service.h

Index: net/dns/dns_config_service.h
diff --git a/net/dns/dns_config_service.h b/net/dns/dns_config_service.h
index 4b77b68c51db353c8f23365aeb99bb994bb9f1b0..02abf3f93f62b60dddf41a00c9bba515474b2ef9 100644
--- a/net/dns/dns_config_service.h
+++ b/net/dns/dns_config_service.h
@@ -136,8 +136,15 @@ class NET_EXPORT_PRIVATE DnsConfigService
   // True if receiver needs to be updated when the config becomes complete.
   bool need_update_;
   // True if the last config sent was empty (instead of |dns_config_|).
+  // Set when |timer_| expires.
   bool last_sent_empty_;
 
+  // Initialized and updated on Invalidate* call.
+  base::TimeTicks last_invalidate_config_time_;
+  base::TimeTicks last_invalidate_hosts_time_;
+  // Initialized and updated when |timer_| expires.
+  base::TimeTicks last_sent_empty_time_;
+
   // Started in Invalidate*, cleared in On*Read.
   base::OneShotTimer<DnsConfigService> timer_;