QUIC - added force_hol_blocking field trial param to enable forced HOL blocking.

net/quic/quic_stream_factory.h

Index: net/quic/quic_stream_factory.h
diff --git a/net/quic/quic_stream_factory.h b/net/quic/quic_stream_factory.h
index 2f014d1242105c7ca6459381dd2b78bda8c7f588..fbb91abb3650f00e02c2d183e2e4f10f41e74381 100644
--- a/net/quic/quic_stream_factory.h
+++ b/net/quic/quic_stream_factory.h
@@ -192,6 +192,7 @@ class NET_EXPORT_PRIVATE QuicStreamFactory
       int idle_connection_timeout_seconds,
       bool migrate_sessions_on_network_change,
       bool migrate_sessions_early,
+      bool force_hol_blocking,
       const QuicTagVector& connection_options,
       bool enable_token_binding);
   ~QuicStreamFactory() override;
@@ -572,6 +573,9 @@ class NET_EXPORT_PRIVATE QuicStreamFactory
   // experiences poor connectivity.
   const bool migrate_sessions_early_;
 
+  // If set, force HOL blocking.  For measurement purposes.
+  const bool force_hol_blocking_;
+
   // Each profile will (probably) have a unique port_seed_ value.  This value
   // is used to help seed a pseudo-random number generator (PortSuggester) so
   // that we consistently (within this profile) suggest the same ephemeral