Make QpackDecoderStreamSender buffer serialized instructions.

QpackEncoderStreamSender has already been modified the same way at cr/273553497.

This way Header Acknowledgement and Insert Count Increment emitted from a single
QpackDecoder::OnDecodingCompleted() call can be coalesced into a single write
without any copying.

gfe-relnote: n/a, change to QUIC v99-only code.  Protected by existing disabled gfe2_reloadable_flag_quic_enable_version_99.
PiperOrigin-RevId: 274280271
Change-Id: I8152636818ef2c0dbbd3b95355c5280cebcd04c6
diff --git a/quic/core/qpack/qpack_decoder_test.cc b/quic/core/qpack/qpack_decoder_test.cc
index faa0f9d..d0ff30f 100644
--- a/quic/core/qpack/qpack_decoder_test.cc
+++ b/quic/core/qpack/qpack_decoder_test.cc
@@ -861,14 +861,14 @@
 
   EXPECT_CALL(handler_, OnHeaderDecoded(Eq("foo"), Eq("bar")));
   EXPECT_CALL(handler_, OnDecodingCompleted());
-  EXPECT_CALL(decoder_stream_sender_delegate_,
-              WriteStreamData(Eq(kHeaderAcknowledgement)));
 
   // Decoder received two insertions, but Header Acknowledgement only increases
   // Known Insert Count to one.  Decoder should send an Insert Count Increment
   // instruction with increment of one to update Known Insert Count to two.
   EXPECT_CALL(decoder_stream_sender_delegate_,
-              WriteStreamData(Eq(QuicTextUtils::HexDecode("01"))));
+              WriteStreamData(Eq(QuicTextUtils::HexDecode(
+                  "81"       // Header Acknowledgement on stream 1
+                  "01"))));  // Insert Count Increment with increment of one
 
   DecodeHeaderBlock(QuicTextUtils::HexDecode(
       "0200"   // Required Insert Count 1 and Delta Base 0.