Add unit test to test user_priv parameter.

Change-Id: I6ba6171e43e0a43331ee0a7b698590b143979c44
diff --git a/test/decode_test_driver.cc b/test/decode_test_driver.cc
index 778c9a3..655b090 100644
--- a/test/decode_test_driver.cc
+++ b/test/decode_test_driver.cc
@@ -24,12 +24,17 @@
 }
 
 vpx_codec_err_t Decoder::DecodeFrame(const uint8_t *cxdata, size_t size) {
+  return DecodeFrame(cxdata, size, NULL);
+}
+
+vpx_codec_err_t Decoder::DecodeFrame(const uint8_t *cxdata, size_t size,
+                                     void *user_priv) {
   vpx_codec_err_t res_dec;
   InitOnce();
   REGISTER_STATE_CHECK(
       res_dec = vpx_codec_decode(&decoder_,
                                  cxdata, static_cast<unsigned int>(size),
-                                 NULL, 0));
+                                 user_priv, 0));
   return res_dec;
 }