Add Cronet experimental option to disable ipv6

net/dns/mapped_host_resolver.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NET_DNS_MAPPED_HOST_RESOLVER_H_
 #define NET_DNS_MAPPED_HOST_RESOLVER_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 39 matching lines @@
               AddressList* addresses,
               const CompletionCallback& callback,
               std::unique_ptr<Request>* request,
               const NetLogWithSource& net_log) override;
   int ResolveFromCache(const RequestInfo& info,
                        AddressList* addresses,
                        const NetLogWithSource& net_log) override;
   void SetDnsClientEnabled(bool enabled) override;
   HostCache* GetHostCache() override;
   std::unique_ptr<base::Value> GetDnsConfigAsValue() const override;
+  void SetDefaultAddressFamily(AddressFamily address_family) override;
+  AddressFamily GetDefaultAddressFamily() const override;
 
  private:
   // Modify the request |info| according to |rules_|. Returns either OK or
   // the network error code that the hostname's resolution mapped to.
   int ApplyRules(RequestInfo* info) const;
 
   std::unique_ptr<HostResolver> impl_;
 
   HostMappingRules rules_;
 };
 
 }  // namespace net
 
 #endif  // NET_DNS_MAPPED_HOST_RESOLVER_H_