Make QuicSpdyStream::WriteTrailers virtual for tests. No behavior change.

net/quic/quic_spdy_stream.h

Index: net/quic/quic_spdy_stream.h
diff --git a/net/quic/quic_spdy_stream.h b/net/quic/quic_spdy_stream.h
index c8d2843ddadec77fa4519268577703f225f3f0a8..cae7b31c51910e87cb7467e4e54a5f1f297c7170 100644
--- a/net/quic/quic_spdy_stream.h
+++ b/net/quic/quic_spdy_stream.h
@@ -99,8 +99,8 @@ class NET_EXPORT_PRIVATE QuicSpdyStream : public ReliableQuicStream {
 
   // Writes the trailers contained in |trailer_block| to the dedicated
   // headers stream. Trailers will always have the FIN set.
-  size_t WriteTrailers(SpdyHeaderBlock trailer_block,
-                       QuicAckListenerInterface* ack_notifier_delegate);
+  virtual size_t WriteTrailers(SpdyHeaderBlock trailer_block,
+                               QuicAckListenerInterface* ack_notifier_delegate);
 
   // Marks |bytes_consumed| of the headers data as consumed.
   void MarkHeadersConsumed(size_t bytes_consumed);