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..99d741b4700dcbcad3eaedb118cef6e9b1b17612 100644
--- a/net/udp/udp_server_socket.cc
+++ b/net/udp/udp_server_socket.cc
@@ -77,6 +77,10 @@ int UDPServerSocket::LeaveGroup(const IPAddressNumber& group_address) const {
   return socket_.LeaveGroup(group_address);
 }
 
+int UDPServerSocket::SetMulticastInterface(uint32 interface_index) {
+  return socket_.SetMulticastInterface(interface_index);
+}
+
 int UDPServerSocket::SetMulticastTimeToLive(int time_to_live) {
   return socket_.SetMulticastTimeToLive(time_to_live);
 }