Pepper: Allow plugins to call PPB_UDP_Socket::SendTo multiple times.

ppapi/api/ppb_udp_socket.idl

Index: ppapi/api/ppb_udp_socket.idl
diff --git a/ppapi/api/ppb_udp_socket.idl b/ppapi/api/ppb_udp_socket.idl
index e4c87b2a1cd5599a71c2e7727bd5d4d370177310..122f7660e8a1cba0589102a7b036c98af358d4f2 100644
--- a/ppapi/api/ppb_udp_socket.idl
+++ b/ppapi/api/ppb_udp_socket.idl
@@ -153,6 +153,9 @@ interface PPB_UDPSocket {
    * been sent; otherwise, an error code from <code>pp_errors.h</code>.
    * <code>PP_ERROR_NOACCESS</code> will be returned if the caller doesn't have
    * required permissions.
+   * <code>PP_ERROR_INPROGRESS</code> will be returned if the socket is busy
+   * sending. The caller should wait until a pending send completes before
+   * retrying.
    */
   int32_t SendTo([in] PP_Resource udp_socket,
                  [in] str_t buffer,