Add QUIC_EXPORT_PRIVATE to ValueSplittingHeaderList::const_iterator.

This is to fix Chromium linking error in Chromium, see
https://chromium-review.googlesource.com/c/chromium/src/+/1586882/1 and
https://ci.chromium.org/p/chromium/builders/try/win_chromium_compile_dbg_ng/281634.

I prepared the same change in QUICHE on a branch:
https://quiche.googlesource.com/quiche/+/roll/245354565
and tested at https://crrev.com/c/1588357 that it passes.

gfe-relnote: n/a.  Linker-related macro only has effect on Windows.
PiperOrigin-RevId: 245783547
Change-Id: Ib6cde66c9c6776ac0b408eccab627ba63025c555

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 2a8cdf0..d142e47 100644
--- a/quic/core/qpack/value_splitting_header_list.h
+++ b/quic/core/qpack/value_splitting_header_list.h
@@ -17,7 +17,7 @@
  public:
   using value_type = spdy::SpdyHeaderBlock::value_type;
 
-  class const_iterator {
+  class QUIC_EXPORT_PRIVATE const_iterator {
    public:
     // |header_list| must outlive this object.
     const_iterator(const spdy::SpdyHeaderBlock* header_list,