Make QpackEncoder::EncodeHeaderList() and FirstPassEncode() take header_list by const ref.

Changing argument type from const* to const&, as discussed on cr/264499016.

gfe-relnote: n/a, change method signatures, no functional change.
PiperOrigin-RevId: 264808104
Change-Id: I84d6c8c6d4446648bd675e6d9cd58fdd50501693
diff --git a/quic/core/http/end_to_end_test.cc b/quic/core/http/end_to_end_test.cc
index 27d3785..6eb6ac6 100644
--- a/quic/core/http/end_to_end_test.cc
+++ b/quic/core/http/end_to_end_test.cc
@@ -2469,7 +2469,7 @@
     qpack_encoder.set_qpack_stream_sender_delegate(
         &encoder_stream_sender_delegate);
     std::string encoded_headers =
-        qpack_encoder.EncodeHeaderList(/* stream_id = */ 0, &headers);
+        qpack_encoder.EncodeHeaderList(/* stream_id = */ 0, headers);
     header_size = encoded_headers.size();
   }