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

net/dns/dns_client.h

Index: net/dns/dns_client.h
diff --git a/net/dns/dns_client.h b/net/dns/dns_client.h
index 650c7d0d4166aee2f0591a4fb0edd6663474bdd0..0484d44c0c60f3f4a320c7215d69f6b440cdf63f 100644
--- a/net/dns/dns_client.h
+++ b/net/dns/dns_client.h
@@ -22,7 +22,8 @@ class NET_EXPORT DnsClient {
  public:
   virtual ~DnsClient() {}
 
-  // Creates a new DnsTransactionFactory according to the new |config|.
+  // Destroys the current DnsTransactionFactory and creates a new one
+  // according to |config|, unless it is invalid or has |unhandled_options|.
   virtual void SetConfig(const DnsConfig& config) = 0;
 
   // Returns NULL if the current config is not valid.