QUIC WINDOW_UPDATE and BLOCKED frames instigate ACKs.

net/quic/quic_connection.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/quic/quic_connection.h"
 
 #include <string.h>
 #include <sys/types.h>
 #include <algorithm>
 #include <iterator>
@@ skipping 824 matching lines @@
 
 QuicStopWaitingFrame* QuicConnection::CreateStopWaitingFrame() {
   QuicStopWaitingFrame stop_waiting;
   UpdateStopWaiting(&stop_waiting);
   return new QuicStopWaitingFrame(stop_waiting);
 }
 
 bool QuicConnection::ShouldLastPacketInstigateAck() const {
   if (!last_stream_frames_.empty() ||
       !last_goaway_frames_.empty() ||
-      !last_rst_frames_.empty()) {
+      !last_rst_frames_.empty() ||
+      !last_window_update_frames_.empty() ||
+      !last_blocked_frames_.empty()) {
     return true;
   }
 
   if (!last_ack_frames_.empty() &&
       last_ack_frames_.back().received_info.is_truncated) {
     return true;
   }
   return false;
 }
 
@@ skipping 924 matching lines @@
   // If we changed the generator's batch state, restore original batch state.
   if (!already_in_batch_mode_) {
     DVLOG(1) << "Leaving Batch Mode.";
     connection_->packet_generator_.FinishBatchOperations();
   }
   DCHECK_EQ(already_in_batch_mode_,
             connection_->packet_generator_.InBatchMode());
 }
 
 }  // namespace net