Add Cronet experimental option to disable ipv6

net/dns/mapped_host_resolver.cc

Index: net/dns/mapped_host_resolver.cc
diff --git a/net/dns/mapped_host_resolver.cc b/net/dns/mapped_host_resolver.cc
index e29c85ea75c4b8bb55352eed3663769b17c4bcf8..39c313c74b33d35feeb2131d75e681edd53ebfc9 100644
--- a/net/dns/mapped_host_resolver.cc
+++ b/net/dns/mapped_host_resolver.cc
@@ -56,6 +56,14 @@ std::unique_ptr<base::Value> MappedHostResolver::GetDnsConfigAsValue() const {
   return impl_->GetDnsConfigAsValue();
 }
 
+void MappedHostResolver::SetDefaultAddressFamily(AddressFamily address_family) {
+  impl_->SetDefaultAddressFamily(address_family);
+}
+
+AddressFamily MappedHostResolver::GetDefaultAddressFamily() const {
+  return impl_->GetDefaultAddressFamily();
+}
+
 int MappedHostResolver::ApplyRules(RequestInfo* info) const {
   HostPortPair host_port(info->host_port_pair());
   if (rules_.RewriteHost(&host_port)) {