Add support of IP_MULTICAST_IF in UDP sockets.

net/udp/datagram_server_socket.h

Index: net/udp/datagram_server_socket.h
diff --git a/net/udp/datagram_server_socket.h b/net/udp/datagram_server_socket.h
index 0561d8a6c7d90db8e4b63aa5ce54e2dea2e027d8..8fe3dca094131919da21c64b0184a7cab03d0dd6 100644
--- a/net/udp/datagram_server_socket.h
+++ b/net/udp/datagram_server_socket.h
@@ -69,16 +69,20 @@ class NET_EXPORT DatagramServerSocket : public DatagramSocket {
   // Should be called before Listen().
   virtual void AllowBroadcast() = 0;
 
+  // Set interface to use for multicast. If |interface_index| set to 0, default
+  // interface is used. Must be called before |JoinGroup| or |LeaveGroup|.
+  virtual int SetMulticastInterface(uint32 interface_index) = 0;
+
   // Join the multicast group with address |group_address|.
   // Returns a network error code.
-  virtual int JoinGroup(const IPAddressNumber& group_address) const = 0;
+  virtual int JoinGroup(const IPAddressNumber& group_address) = 0;
 
   // Leave the multicast group with address |group_address|.
   // If the socket hasn't joined the group, it will be ignored.
   // It's optional to leave the multicast group before destroying
   // the socket. It will be done by the OS.
   // Returns a network error code.
-  virtual int LeaveGroup(const IPAddressNumber& group_address) const = 0;
+  virtual int LeaveGroup(const IPAddressNumber& group_address) = 0;
 
   // Set the time-to-live option for UDP packets sent to the multicast
   // group address. The default value of this option is 1.