Add reverse dns lookup as a method on InternetAddress.

runtime/bin/socket.h

Index: runtime/bin/socket.h
diff --git a/runtime/bin/socket.h b/runtime/bin/socket.h
index 0b0577363581e3afe446dc6d707d95bd2a64bf50..386f2314e2ee3c956b4e0108c4ed3dd57748d23a 100644
--- a/runtime/bin/socket.h
+++ b/runtime/bin/socket.h
@@ -150,6 +150,7 @@ class Socket {
   enum SocketRequest {
     kLookupRequest = 0,
     kListInterfacesRequest = 1,
+    kReverseLookupRequest = 2,
   };
 
   static bool Initialize();
@@ -174,6 +175,7 @@ class Socket {
   static AddressList<SocketAddress>* LookupAddress(const char* host,
                                                    int type,
                                                    OSError** os_error);
+  static const char* ReverseLookup(RawAddr addr, OSError** os_error);
 
   // List interfaces. Returns a AddressList of InterfaceSocketAddress's.
   static AddressList<InterfaceSocketAddress>* ListInterfaces(