Update IETF QUIC Stop Sending to use varint for error code

Recent update to the IETF QUIC specification changed the serialization of
the Stop Sending frame's error code from uint16_t to varint ... this CL
implements that change.

Note that we still keep the error code as a uint16_t -- a later CL will update
the internal storage of error codes to uint64_t.

gfe-relnote: N/A change to IETF QUIC (V99) only.
PiperOrigin-RevId: 258404381
Change-Id: I4872cd5fce68454978880a8009c2de7ada43bdf8
diff --git a/quic/core/quic_framer_test.cc b/quic/core/quic_framer_test.cc
index 29ae36f..95b9187 100644
--- a/quic/core/quic_framer_test.cc
+++ b/quic/core/quic_framer_test.cc
@@ -11650,7 +11650,7 @@
       {"Unable to read stop sending stream id.",
        {kVarInt62FourBytes + 0x01, 0x02, 0x03, 0x04}},
       {"Unable to read stop sending application error code.",
-       {0x76, 0x54}},
+       {kVarInt62FourBytes + 0x00, 0x00, 0x76, 0x54}},
   };
   // clang-format on
 
@@ -11701,7 +11701,7 @@
     // Stream ID
     kVarInt62FourBytes + 0x01, 0x02, 0x03, 0x04,
     // Application error code
-    0xff, 0xff
+    kVarInt62FourBytes + 0x00, 0x00, 0xff, 0xff
   };
   // clang-format on