Change the Hyperloop client initial stream flow control window to 16KB, but then raise it to 4MB in the stream itself.

This is in preparation for fixing the dynamic flow control window logic in a follow up. Breaks out some of the code protected by gfe2_restart_flag_track_hyperloop_streams4 into a new flag. gfe2_restart_flag_track_hyperloop_streams4 will remain disabled until this new flag is removed.

Protected by gfe2_restart_flag_move_hyperloop_stream_flow_control.

PiperOrigin-RevId: 769812846

quiche/quic/core/quic_stream.h

diff --git a/quiche/quic/core/quic_stream.h b/quiche/quic/core/quic_stream.h
index 4a7b738..0636b54 100644
--- a/quiche/quic/core/quic_stream.h
+++ b/quiche/quic/core/quic_stream.h
@@ -293,6 +293,9 @@
   QuicStreamOffset NumBytesConsumed() const {
     return sequencer()->NumBytesConsumed();
   }
+  QuicStreamOffset send_window_offset() const {
+    return flow_controller_->send_window_offset();
+  }
 
   // Called when endpoint receives a frame which could increase the highest
   // offset.