Add QUIC_EXPORT_PRIVATE to prepare for presubmit

A subsequent CL (cl/276207410) will add a presubmit to mitigate QUICHE merge issues, this CL gets our code-base to a state of compliance with that presubmit. These haven't caused issues in Chromium if they were only used in the same build unit, but it's worth it to have a consistent use across the codebase to make sure everything works in Chromium.

gfe-relnote: no behavior change, QUIC_EXPORT_PRIVATE is a no-op in google3
PiperOrigin-RevId: 276276729
Change-Id: I67ab8c9df92d24d0006de7084ca563153f0bf07f
diff --git a/quic/core/quic_stream_send_buffer.h b/quic/core/quic_stream_send_buffer.h
index 74e9d0d..f3b799c 100644
--- a/quic/core/quic_stream_send_buffer.h
+++ b/quic/core/quic_stream_send_buffer.h
@@ -26,7 +26,7 @@
 // contiguous memory space. Please note, BufferedSlice is constructed when
 // stream data is saved in send buffer and is removed when stream data is fully
 // acked. It is move-only.
-struct BufferedSlice {
+struct QUIC_EXPORT_PRIVATE BufferedSlice {
   BufferedSlice(QuicMemSlice mem_slice, QuicStreamOffset offset);
   BufferedSlice(BufferedSlice&& other);
   BufferedSlice& operator=(BufferedSlice&& other);
@@ -41,7 +41,7 @@
   QuicStreamOffset offset;
 };
 
-struct StreamPendingRetransmission {
+struct QUIC_EXPORT_PRIVATE StreamPendingRetransmission {
   StreamPendingRetransmission(QuicStreamOffset offset, QuicByteCount length)
       : offset(offset), length(length) {}