Add support of IP_MULTICAST_IF in UDP sockets.

net/udp/udp_server_socket.cc

Index: net/udp/udp_server_socket.cc
diff --git a/net/udp/udp_server_socket.cc b/net/udp/udp_server_socket.cc
index 79086f8a1998e90f4a7b51c3c3f09779d7721983..5356d0564083f8551137dcbef87541a1721d1921 100644
--- a/net/udp/udp_server_socket.cc
+++ b/net/udp/udp_server_socket.cc
@@ -69,11 +69,15 @@ void UDPServerSocket::AllowBroadcast() {
   socket_.AllowBroadcast();
 }
 
-int UDPServerSocket::JoinGroup(const IPAddressNumber& group_address) const {
+int UDPServerSocket::SetMulticastInterface(uint32 interface_index) {
+  return socket_.SetMulticastInterface(interface_index);
+}
+
+int UDPServerSocket::JoinGroup(const IPAddressNumber& group_address) {
   return socket_.JoinGroup(group_address);
 }
 
-int UDPServerSocket::LeaveGroup(const IPAddressNumber& group_address) const {
+int UDPServerSocket::LeaveGroup(const IPAddressNumber& group_address) {
   return socket_.LeaveGroup(group_address);
 }