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

net/dns/dns_client.cc

Index: net/dns/dns_client.cc
diff --git a/net/dns/dns_client.cc b/net/dns/dns_client.cc
index 976f1533905112d21097fff65187eafc973e1224..9e29ca4265d0a5c2332cbfcb0e371bc5d222f56c 100644
--- a/net/dns/dns_client.cc
+++ b/net/dns/dns_client.cc
@@ -27,7 +27,7 @@ class DnsClientImpl : public DnsClient {
   virtual void SetConfig(const DnsConfig& config) OVERRIDE {
     factory_.reset();
     session_ = NULL;
-    if (config.IsValid()) {
+    if (config.IsValid() && !config.unhandled_options) {

[mmenke, 2013/08/16 17:35:39]

Is the second check needed?  We always return an error when we set
config.unhandled_options to true, so it seems like we could just DCHECK on it
here.  Or am I missing something?

[szym, 2013/08/16 17:48:27]

We still want to detect DNS changes, and for that DnsConfigService must not
fail. This CL introduces "unhandled_options" to be separate from whether
DnsConfigService is working. "unhandled_options = true" means, we got a
DnsConfig, but our DnsClient cannot fully execute it.

       ClientSocketFactory* factory = ClientSocketFactory::GetDefaultFactory();
       scoped_ptr<DnsSocketPool> socket_pool(
           config.randomize_ports ? DnsSocketPool::CreateDefault(factory)