Detect domain-specific resolvers on OS X and disable DnsClient in such cases.

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 10631773291bbe2886d5a6431f5f1ef058d1ed0d..dfa10cf9c6c4c78744b29b43af0041ca3796c92f 100644
--- a/net/dns/host_resolver_impl.cc
+++ b/net/dns/host_resolver_impl.cc
@@ -2133,7 +2133,7 @@ void HostResolverImpl::OnDNSChanged() {
   // the newly started jobs use the new config.
   if (dns_client_.get()) {
     dns_client_->SetConfig(dns_config);
-    if (dns_config.IsValid())
+    if (dns_client_->GetConfig())
       UMA_HISTOGRAM_BOOLEAN("AsyncDNS.DnsClientEnabled", true);
   }
 
@@ -2199,7 +2199,7 @@ void HostResolverImpl::SetDnsClient(scoped_ptr<DnsClient> dns_client) {
   NetworkChangeNotifier::GetDnsConfig(&dns_config);
   dns_client_->SetConfig(dns_config);
   num_dns_failures_ = 0;
-  if (dns_config.IsValid())
+  if (dns_client_->GetConfig())
     UMA_HISTOGRAM_BOOLEAN("AsyncDNS.DnsClientEnabled", true);
 }