Change -Wdefaulted-function-deleted fix in ValueSplittingHeaderList::const_iterator

After looking at cl/245511621 some more, I believe we can keep header_list_ const by deleting the copy operator since it is never used.

Compile-only change.
PiperOrigin-RevId: 245859533
Change-Id: I40b36423ffba526ffdc7cf6a9ae4fec10a0c7cc3

quic/core/qpack/value_splitting_header_list.h

diff --git a/quic/core/qpack/value_splitting_header_list.h b/quic/core/qpack/value_splitting_header_list.h
index d142e47..8d994ae 100644
--- a/quic/core/qpack/value_splitting_header_list.h
+++ b/quic/core/qpack/value_splitting_header_list.h
@@ -23,7 +23,7 @@
     const_iterator(const spdy::SpdyHeaderBlock* header_list,
                    spdy::SpdyHeaderBlock::const_iterator header_list_iterator);
     const_iterator(const const_iterator&) = default;
-    const_iterator& operator=(const const_iterator&) = default;
+    const_iterator& operator=(const const_iterator&) = delete;
 
     bool operator==(const const_iterator& other) const;
     bool operator!=(const const_iterator& other) const;
@@ -37,7 +37,7 @@
     // Find next '\0' character; update |value_end_| and |header_field_|.
     void UpdateHeaderField();
 
-    const spdy::SpdyHeaderBlock* header_list_;
+    const spdy::SpdyHeaderBlock* const header_list_;
     spdy::SpdyHeaderBlock::const_iterator header_list_iterator_;
     QuicStringPiece::size_type value_start_;
     QuicStringPiece::size_type value_end_;