I think I understand the issue.

This test probably started failing with PlzNavigate since this series of CL:
https://codereview.chromium.org/2510333002

|encoded_data_length| stopped being sent in OnReceivedData, but is now sent separately since:
https://codereview.chromium.org/2519893002
This doc is interesting: https://docs.google.com/document/d/1Q-LpTPcV8QApeXSL24MVZHjP-uQK7RAK300t_qZaGFQ/edit#heading=h.cq4slq8hi7q7

With PlzNavigate, it is not sent.
As far as I understand, we probably need the async_resource_handler to send OnTransferSizeUpdated messages.
I found a CL that does exactly what I would like to do, but in the mojo_async_resource_handler:
https://codereview.chromium.org/2566943002
+CC yhirano: FYI
I will probably try to do something similar.

Yes, you need to send the message. But I don't understand what you mean by "we probably need the async_resource_handler to send OnTransferSizeUpdated messages". Are you using mojo on content::AsyncResourceHandler?

Yes you are right. I was not aware that LoadingWithMojo was enabled by default. Thanks!
So I have a much simpler explanation for this bug:

In MojoAsyncResourceHandler::OnReadCompleted():
```
  const ResourceRequestInfoImpl* info = GetRequestInfo();
  if (info->ShouldReportRawHeaders()) {
    auto transfer_size_diff = CalculateRecentlyReceivedBytes();
    if (transfer_size_diff > 0)
      url_loader_client_->OnTransferSizeUpdated(transfer_size_diff);
  }

```

OnTransferSizeUpdated is sent only if ShouldReportRawHeaders is enabled. It is enabled in InspectorNetworkAgent::WillSendRequest(). The latter is not called before the request is sent with PlzNavigate.

It means this bug is a duplicate of  issue 760924 .